drivers/scsi/atari_scsi.c

/*
 * atari_scsi.c -- Device dependent functions for the Atari generic SCSI port
 *
 * Copyright 1994 Roman Hodek <Roman.Hodek@informatik.uni-erlangen.de>
 *
 *   Loosely based on the work of Robert De Vries' team and added:
 *    - working real DMA
 *    - Falcon support (untested yet!)   ++bjoern fixed and now it works
 *    - lots of extensions and bug fixes.
 *
 * This file is subject to the terms and conditions of the GNU General Public
 * License.  See the file COPYING in the main directory of this archive
 * for more details.
 *
 */


/**************************************************************************/
/*                                                                        */
/* Notes for Falcon SCSI:                                                 */
/* ----------------------                                                 */
/*                                                                        */
/* Since the Falcon SCSI uses the ST-DMA chip, that is shared among       */
/* several device drivers, locking and unlocking the access to this       */
/* chip is required. But locking is not possible from an interrupt,       */
/* since it puts the process to sleep if the lock is not available.       */
/* This prevents "late" locking of the DMA chip, i.e. locking it just     */
/* before using it, since in case of disconnection-reconnection           */
/* commands, the DMA is started from the reselection interrupt.           */
/*                                                                        */
/* Two possible schemes for ST-DMA-locking would be:                      */
/*  1) The lock is taken for each command separately and disconnecting    */
/*     is forbidden (i.e. can_queue = 1).                                 */
/*  2) The DMA chip is locked when the first command comes in and         */
/*     released when the last command is finished and all queues are      */
/*     empty.                                                             */
/* The first alternative would result in bad performance, since the       */
/* interleaving of commands would not be used. The second is unfair to    */
/* other drivers using the ST-DMA, because the queues will seldom be      */
/* totally empty if there is a lot of disk traffic.                       */
/*                                                                        */
/* For this reasons I decided to employ a more elaborate scheme:          */
/*  - First, we give up the lock every time we can (for fairness), this    */
/*    means every time a command finishes and there are no other commands */
/*    on the disconnected queue.                                          */
/*  - If there are others waiting to lock the DMA chip, we stop           */
/*    issuing commands, i.e. moving them onto the issue queue.           */
/*    Because of that, the disconnected queue will run empty in a         */
/*    while. Instead we go to sleep on a 'fairness_queue'.                */
/*  - If the lock is released, all processes waiting on the fairness      */
/*    queue will be woken. The first of them tries to re-lock the DMA,     */
/*    the others wait for the first to finish this task. After that,      */
/*    they can all run on and do their commands...                        */
/* This sounds complicated (and it is it :-(), but it seems to be a       */
/* good compromise between fairness and performance: As long as no one     */
/* else wants to work with the ST-DMA chip, SCSI can go along as          */
/* usual. If now someone else comes, this behaviour is changed to a       */
/* "fairness mode": just already initiated commands are finished and      */
/* then the lock is released. The other one waiting will probably win     */
/* the race for locking the DMA, since it was waiting for longer. And     */
/* after it has finished, SCSI can go ahead again. Finally: I hope I      */
/* have not produced any deadlock possibilities!                          */
/*                                                                        */
/**************************************************************************/


#include <linux/module.h>
#include <linux/types.h>
#include <linux/delay.h>
#include <linux/blkdev.h>
#include <linux/interrupt.h>
#include <linux/init.h>
#include <linux/nvram.h>
#include <linux/bitops.h>
#include <linux/wait.h>
#include <linux/platform_device.h>

#include <asm/setup.h>
#include <asm/atarihw.h>
#include <asm/atariints.h>
#include <asm/atari_stdma.h>
#include <asm/atari_stram.h>
#include <asm/io.h>

#include <scsi/scsi_host.h>

/* Definitions for the core NCR5380 driver. */

#define REAL_DMA
#define SUPPORT_TAGS
#define MAX_TAGS                        32

#define NCR5380_implementation_fields   /* none */

#define NCR5380_read(reg)               atari_scsi_reg_read(reg)
#define NCR5380_write(reg, value)       atari_scsi_reg_write(reg, value)

#define NCR5380_queue_command           atari_scsi_queue_command
#define NCR5380_abort                   atari_scsi_abort
#define NCR5380_show_info               atari_scsi_show_info
#define NCR5380_info                    atari_scsi_info

#define NCR5380_dma_read_setup(instance, data, count) \
        atari_scsi_dma_setup(instance, data, count, 0)
#define NCR5380_dma_write_setup(instance, data, count) \
        atari_scsi_dma_setup(instance, data, count, 1)
#define NCR5380_dma_residual(instance) \
        atari_scsi_dma_residual(instance)
#define NCR5380_dma_xfer_len(instance, cmd, phase) \
        atari_dma_xfer_len(cmd->SCp.this_residual, cmd, !((phase) & SR_IO))

#define NCR5380_acquire_dma_irq(instance)      falcon_get_lock()
#define NCR5380_release_dma_irq(instance)      falcon_release_lock()

#include "NCR5380.h"


#define	IS_A_TT()	ATARIHW_PRESENT(TT_SCSI)

#define	SCSI_DMA_WRITE_P(elt,val)				\
	do {							\
		unsigned long v = val;				\
		tt_scsi_dma.elt##_lo = v & 0xff;		\
		v >>= 8;					\
		tt_scsi_dma.elt##_lmd = v & 0xff;		\
		v >>= 8;					\
		tt_scsi_dma.elt##_hmd = v & 0xff;		\
		v >>= 8;					\
		tt_scsi_dma.elt##_hi = v & 0xff;		\
	} while(0)

#define	SCSI_DMA_READ_P(elt)					\
	(((((((unsigned long)tt_scsi_dma.elt##_hi << 8) |	\
	     (unsigned long)tt_scsi_dma.elt##_hmd) << 8) |	\
	   (unsigned long)tt_scsi_dma.elt##_lmd) << 8) |	\
	 (unsigned long)tt_scsi_dma.elt##_lo)


static inline void SCSI_DMA_SETADR(unsigned long adr)
{
	st_dma.dma_lo = (unsigned char)adr;
	MFPDELAY();
	adr >>= 8;
	st_dma.dma_md = (unsigned char)adr;
	MFPDELAY();
	adr >>= 8;
	st_dma.dma_hi = (unsigned char)adr;
	MFPDELAY();
}

static inline unsigned long SCSI_DMA_GETADR(void)
{
	unsigned long adr;
	adr = st_dma.dma_lo;
	MFPDELAY();
	adr |= (st_dma.dma_md & 0xff) << 8;
	MFPDELAY();
	adr |= (st_dma.dma_hi & 0xff) << 16;
	MFPDELAY();
	return adr;
}

#define HOSTDATA_DMALEN		(((struct NCR5380_hostdata *) \
				(atari_scsi_host->hostdata))->dma_len)

/* Time (in jiffies) to wait after a reset; the SCSI standard calls for 250ms,
 * we usually do 0.5s to be on the safe side. But Toshiba CD-ROMs once more
 * need ten times the standard value... */
#ifndef CONFIG_ATARI_SCSI_TOSHIBA_DELAY
#define	AFTER_RESET_DELAY	(HZ/2)
#else
#define	AFTER_RESET_DELAY	(5*HZ/2)
#endif

#ifdef REAL_DMA
static void atari_scsi_fetch_restbytes(void);
#endif

static struct Scsi_Host *atari_scsi_host;
static unsigned char (*atari_scsi_reg_read)(unsigned char reg);
static void (*atari_scsi_reg_write)(unsigned char reg, unsigned char value);

#ifdef REAL_DMA
static unsigned long	atari_dma_residual, atari_dma_startaddr;
static short		atari_dma_active;
/* pointer to the dribble buffer */
static char		*atari_dma_buffer;
/* precalculated physical address of the dribble buffer */
static unsigned long	atari_dma_phys_buffer;
/* != 0 tells the Falcon int handler to copy data from the dribble buffer */
static char		*atari_dma_orig_addr;
/* size of the dribble buffer; 4k seems enough, since the Falcon cannot use
 * scatter-gather anyway, so most transfers are 1024 byte only. In the rare
 * cases where requests to physical contiguous buffers have been merged, this
 * request is <= 4k (one page). So I don't think we have to split transfers
 * just due to this buffer size...
 */
#define	STRAM_BUFFER_SIZE	(4096)
/* mask for address bits that can't be used with the ST-DMA */
static unsigned long	atari_dma_stram_mask;
#define STRAM_ADDR(a)	(((a) & atari_dma_stram_mask) == 0)
#endif

static int setup_can_queue = -1;
module_param(setup_can_queue, int, 0);
static int setup_cmd_per_lun = -1;
module_param(setup_cmd_per_lun, int, 0);
static int setup_sg_tablesize = -1;
module_param(setup_sg_tablesize, int, 0);
#ifdef SUPPORT_TAGS
static int setup_use_tagged_queuing = -1;
module_param(setup_use_tagged_queuing, int, 0);
#endif
static int setup_hostid = -1;
module_param(setup_hostid, int, 0);


#if defined(REAL_DMA)

static int scsi_dma_is_ignored_buserr(unsigned char dma_stat)
{
	int i;
	unsigned long addr = SCSI_DMA_READ_P(dma_addr), end_addr;

	if (dma_stat & 0x01) {

		/* A bus error happens when DMA-ing from the last page of a
		 * physical memory chunk (DMA prefetch!), but that doesn't hurt.
		 * Check for this case:
		 */

		for (i = 0; i < m68k_num_memory; ++i) {
			end_addr = m68k_memory[i].addr + m68k_memory[i].size;
			if (end_addr <= addr && addr <= end_addr + 4)
				return 1;
		}
	}
	return 0;
}


#if 0
/* Dead code... wasn't called anyway :-) and causes some trouble, because at
 * end-of-DMA, both SCSI ints are triggered simultaneously, so the NCR int has
 * to clear the DMA int pending bit before it allows other level 6 interrupts.
 */
static void scsi_dma_buserr(int irq, void *dummy)
{
	unsigned char dma_stat = tt_scsi_dma.dma_ctrl;

	/* Don't do anything if a NCR interrupt is pending. Probably it's just
	 * masked... */
	if (atari_irq_pending(IRQ_TT_MFP_SCSI))
		return;

	printk("Bad SCSI DMA interrupt! dma_addr=0x%08lx dma_stat=%02x dma_cnt=%08lx\n",
	       SCSI_DMA_READ_P(dma_addr), dma_stat, SCSI_DMA_READ_P(dma_cnt));
	if (dma_stat & 0x80) {
		if (!scsi_dma_is_ignored_buserr(dma_stat))
			printk("SCSI DMA bus error -- bad DMA programming!\n");
	} else {
		/* Under normal circumstances we never should get to this point,
		 * since both interrupts are triggered simultaneously and the 5380
		 * int has higher priority. When this irq is handled, that DMA
		 * interrupt is cleared. So a warning message is printed here.
		 */
		printk("SCSI DMA intr ?? -- this shouldn't happen!\n");
	}
}
#endif

#endif


static irqreturn_t scsi_tt_intr(int irq, void *dummy)
{
#ifdef REAL_DMA
	int dma_stat;

	dma_stat = tt_scsi_dma.dma_ctrl;

	dprintk(NDEBUG_INTR, "scsi%d: NCR5380 interrupt, DMA status = %02x\n",
		   atari_scsi_host->host_no, dma_stat & 0xff);

	/* Look if it was the DMA that has interrupted: First possibility
	 * is that a bus error occurred...
	 */
	if (dma_stat & 0x80) {
		if (!scsi_dma_is_ignored_buserr(dma_stat)) {
			printk(KERN_ERR "SCSI DMA caused bus error near 0x%08lx\n",
			       SCSI_DMA_READ_P(dma_addr));
			printk(KERN_CRIT "SCSI DMA bus error -- bad DMA programming!");
		}
	}

	/* If the DMA is active but not finished, we have the case
	 * that some other 5380 interrupt occurred within the DMA transfer.
	 * This means we have residual bytes, if the desired end address
	 * is not yet reached. Maybe we have to fetch some bytes from the
	 * rest data register, too. The residual must be calculated from
	 * the address pointer, not the counter register, because only the
	 * addr reg counts bytes not yet written and pending in the rest
	 * data reg!
	 */
	if ((dma_stat & 0x02) && !(dma_stat & 0x40)) {
		atari_dma_residual = HOSTDATA_DMALEN - (SCSI_DMA_READ_P(dma_addr) - atari_dma_startaddr);

		dprintk(NDEBUG_DMA, "SCSI DMA: There are %ld residual bytes.\n",
			   atari_dma_residual);

		if ((signed int)atari_dma_residual < 0)
			atari_dma_residual = 0;
		if ((dma_stat & 1) == 0) {
			/*
			 * After read operations, we maybe have to
			 * transport some rest bytes
			 */
			atari_scsi_fetch_restbytes();
		} else {
			/*
			 * There seems to be a nasty bug in some SCSI-DMA/NCR
			 * combinations: If a target disconnects while a write
			 * operation is going on, the address register of the
			 * DMA may be a few bytes farer than it actually read.
			 * This is probably due to DMA prefetching and a delay
			 * between DMA and NCR.  Experiments showed that the
			 * dma_addr is 9 bytes to high, but this could vary.
			 * The problem is, that the residual is thus calculated
			 * wrong and the next transfer will start behind where
			 * it should.  So we round up the residual to the next
			 * multiple of a sector size, if it isn't already a
			 * multiple and the originally expected transfer size
			 * was.  The latter condition is there to ensure that
			 * the correction is taken only for "real" data
			 * transfers and not for, e.g., the parameters of some
			 * other command.  These shouldn't disconnect anyway.
			 */
			if (atari_dma_residual & 0x1ff) {
				dprintk(NDEBUG_DMA, "SCSI DMA: DMA bug corrected, "
					   "difference %ld bytes\n",
					   512 - (atari_dma_residual & 0x1ff));
				atari_dma_residual = (atari_dma_residual + 511) & ~0x1ff;
			}
		}
		tt_scsi_dma.dma_ctrl = 0;
	}

	/* If the DMA is finished, fetch the rest bytes and turn it off */
	if (dma_stat & 0x40) {
		atari_dma_residual = 0;
		if ((dma_stat & 1) == 0)
			atari_scsi_fetch_restbytes();
		tt_scsi_dma.dma_ctrl = 0;
	}

#endif /* REAL_DMA */

	NCR5380_intr(irq, dummy);

	return IRQ_HANDLED;
}


static irqreturn_t scsi_falcon_intr(int irq, void *dummy)
{
#ifdef REAL_DMA
	int dma_stat;

	/* Turn off DMA and select sector counter register before
	 * accessing the status register (Atari recommendation!)
	 */
	st_dma.dma_mode_status = 0x90;
	dma_stat = st_dma.dma_mode_status;

	/* Bit 0 indicates some error in the DMA process... don't know
	 * what happened exactly (no further docu).
	 */
	if (!(dma_stat & 0x01)) {
		/* DMA error */
		printk(KERN_CRIT "SCSI DMA error near 0x%08lx!\n", SCSI_DMA_GETADR());
	}

	/* If the DMA was active, but now bit 1 is not clear, it is some
	 * other 5380 interrupt that finishes the DMA transfer. We have to
	 * calculate the number of residual bytes and give a warning if
	 * bytes are stuck in the ST-DMA fifo (there's no way to reach them!)
	 */
	if (atari_dma_active && (dma_stat & 0x02)) {
		unsigned long transferred;

		transferred = SCSI_DMA_GETADR() - atari_dma_startaddr;
		/* The ST-DMA address is incremented in 2-byte steps, but the
		 * data are written only in 16-byte chunks. If the number of
		 * transferred bytes is not divisible by 16, the remainder is
		 * lost somewhere in outer space.
		 */
		if (transferred & 15)
			printk(KERN_ERR "SCSI DMA error: %ld bytes lost in "
			       "ST-DMA fifo\n", transferred & 15);

		atari_dma_residual = HOSTDATA_DMALEN - transferred;
		dprintk(NDEBUG_DMA, "SCSI DMA: There are %ld residual bytes.\n",
			   atari_dma_residual);
	} else
		atari_dma_residual = 0;
	atari_dma_active = 0;

	if (atari_dma_orig_addr) {
		/* If the dribble buffer was used on a read operation, copy the DMA-ed
		 * data to the original destination address.
		 */
		memcpy(atari_dma_orig_addr, phys_to_virt(atari_dma_startaddr),
		       HOSTDATA_DMALEN - atari_dma_residual);
		atari_dma_orig_addr = NULL;
	}

#endif /* REAL_DMA */

	NCR5380_intr(irq, dummy);
	return IRQ_HANDLED;
}


#ifdef REAL_DMA
static void atari_scsi_fetch_restbytes(void)
{
	int nr;
	char *src, *dst;
	unsigned long phys_dst;

	/* fetch rest bytes in the DMA register */
	phys_dst = SCSI_DMA_READ_P(dma_addr);
	nr = phys_dst & 3;
	if (nr) {
		/* there are 'nr' bytes left for the last long address
		   before the DMA pointer */
		phys_dst ^= nr;
		dprintk(NDEBUG_DMA, "SCSI DMA: there are %d rest bytes for phys addr 0x%08lx",
			   nr, phys_dst);
		/* The content of the DMA pointer is a physical address!  */
		dst = phys_to_virt(phys_dst);
		dprintk(NDEBUG_DMA, " = virt addr %p\n", dst);
		for (src = (char *)&tt_scsi_dma.dma_restdata; nr != 0; --nr)
			*dst++ = *src++;
	}
}
#endif /* REAL_DMA */


/* This function releases the lock on the DMA chip if there is no
 * connected command and the disconnected queue is empty.
 */

static void falcon_release_lock(void)
{
	if (IS_A_TT())
		return;

	if (stdma_is_locked_by(scsi_falcon_intr))
		stdma_release();
}

/* This function manages the locking of the ST-DMA.
 * If the DMA isn't locked already for SCSI, it tries to lock it by
 * calling stdma_lock(). But if the DMA is locked by the SCSI code and
 * there are other drivers waiting for the chip, we do not issue the
 * command immediately but tell the SCSI mid-layer to defer.
 */

static int falcon_get_lock(void)
{
	if (IS_A_TT())
		return 1;

	if (in_interrupt())
		return stdma_try_lock(scsi_falcon_intr, NULL);

	stdma_lock(scsi_falcon_intr, NULL);
	return 1;
}

#ifndef MODULE
static int __init atari_scsi_setup(char *str)
{
	/* Format of atascsi parameter is:
	 *   atascsi=<can_queue>,<cmd_per_lun>,<sg_tablesize>,<hostid>,<use_tags>
	 * Defaults depend on TT or Falcon, determined at run time.
	 * Negative values mean don't change.
	 */
	int ints[6];

	get_options(str, ARRAY_SIZE(ints), ints);

	if (ints[0] < 1) {
		printk("atari_scsi_setup: no arguments!\n");
		return 0;
	}
	if (ints[0] >= 1)
		setup_can_queue = ints[1];
	if (ints[0] >= 2)
		setup_cmd_per_lun = ints[2];
	if (ints[0] >= 3)
		setup_sg_tablesize = ints[3];
	if (ints[0] >= 4)
		setup_hostid = ints[4];
#ifdef SUPPORT_TAGS
	if (ints[0] >= 5)
		setup_use_tagged_queuing = ints[5];
#endif

	return 1;
}

__setup("atascsi=", atari_scsi_setup);
#endif /* !MODULE */


#ifdef CONFIG_ATARI_SCSI_RESET_BOOT
static void __init atari_scsi_reset_boot(void)
{
	unsigned long end;

	/*
	 * Do a SCSI reset to clean up the bus during initialization. No messing
	 * with the queues, interrupts, or locks necessary here.
	 */

	printk("Atari SCSI: resetting the SCSI bus...");

	/* get in phase */
	NCR5380_write(TARGET_COMMAND_REG,
		      PHASE_SR_TO_TCR(NCR5380_read(STATUS_REG)));

	/* assert RST */
	NCR5380_write(INITIATOR_COMMAND_REG, ICR_BASE | ICR_ASSERT_RST);
	/* The min. reset hold time is 25us, so 40us should be enough */
	udelay(50);
	/* reset RST and interrupt */
	NCR5380_write(INITIATOR_COMMAND_REG, ICR_BASE);
	NCR5380_read(RESET_PARITY_INTERRUPT_REG);

	end = jiffies + AFTER_RESET_DELAY;
	while (time_before(jiffies, end))
		barrier();

	printk(" done\n");
}
#endif

#if defined(REAL_DMA)

static unsigned long atari_scsi_dma_setup(struct Scsi_Host *instance,
					  void *data, unsigned long count,
					  int dir)
{
	unsigned long addr = virt_to_phys(data);

	dprintk(NDEBUG_DMA, "scsi%d: setting up dma, data = %p, phys = %lx, count = %ld, "
		   "dir = %d\n", instance->host_no, data, addr, count, dir);

	if (!IS_A_TT() && !STRAM_ADDR(addr)) {
		/* If we have a non-DMAable address on a Falcon, use the dribble
		 * buffer; 'orig_addr' != 0 in the read case tells the interrupt
		 * handler to copy data from the dribble buffer to the originally
		 * wanted address.
		 */
		if (dir)
			memcpy(atari_dma_buffer, data, count);
		else
			atari_dma_orig_addr = data;
		addr = atari_dma_phys_buffer;
	}

	atari_dma_startaddr = addr;	/* Needed for calculating residual later. */

	/* Cache cleanup stuff: On writes, push any dirty cache out before sending
	 * it to the peripheral. (Must be done before DMA setup, since at least
	 * the ST-DMA begins to fill internal buffers right after setup. For
	 * reads, invalidate any cache, may be altered after DMA without CPU
	 * knowledge.
	 *
	 * ++roman: For the Medusa, there's no need at all for that cache stuff,
	 * because the hardware does bus snooping (fine!).
	 */
	dma_cache_maintenance(addr, count, dir);

	if (count == 0)
		printk(KERN_NOTICE "SCSI warning: DMA programmed for 0 bytes !\n");

	if (IS_A_TT()) {
		tt_scsi_dma.dma_ctrl = dir;
		SCSI_DMA_WRITE_P(dma_addr, addr);
		SCSI_DMA_WRITE_P(dma_cnt, count);
		tt_scsi_dma.dma_ctrl = dir | 2;
	} else { /* ! IS_A_TT */

		/* set address */
		SCSI_DMA_SETADR(addr);

		/* toggle direction bit to clear FIFO and set DMA direction */
		dir <<= 8;
		st_dma.dma_mode_status = 0x90 | dir;
		st_dma.dma_mode_status = 0x90 | (dir ^ 0x100);
		st_dma.dma_mode_status = 0x90 | dir;
		udelay(40);
		/* On writes, round up the transfer length to the next multiple of 512
		 * (see also comment at atari_dma_xfer_len()). */
		st_dma.fdc_acces_seccount = (count + (dir ? 511 : 0)) >> 9;
		udelay(40);
		st_dma.dma_mode_status = 0x10 | dir;
		udelay(40);
		/* need not restore value of dir, only boolean value is tested */
		atari_dma_active = 1;
	}

	return count;
}


static long atari_scsi_dma_residual(struct Scsi_Host *instance)
{
	return atari_dma_residual;
}


#define	CMD_SURELY_BLOCK_MODE	0
#define	CMD_SURELY_BYTE_MODE	1
#define	CMD_MODE_UNKNOWN		2

static int falcon_classify_cmd(struct scsi_cmnd *cmd)
{
	unsigned char opcode = cmd->cmnd[0];

	if (opcode == READ_DEFECT_DATA || opcode == READ_LONG ||
	    opcode == READ_BUFFER)
		return CMD_SURELY_BYTE_MODE;
	else if (opcode == READ_6 || opcode == READ_10 ||
		 opcode == 0xa8 /* READ_12 */ || opcode == READ_REVERSE ||
		 opcode == RECOVER_BUFFERED_DATA) {
		/* In case of a sequential-access target (tape), special care is
		 * needed here: The transfer is block-mode only if the 'fixed' bit is
		 * set! */
		if (cmd->device->type == TYPE_TAPE && !(cmd->cmnd[1] & 1))
			return CMD_SURELY_BYTE_MODE;
		else
			return CMD_SURELY_BLOCK_MODE;
	} else
		return CMD_MODE_UNKNOWN;
}


/* This function calculates the number of bytes that can be transferred via
 * DMA. On the TT, this is arbitrary, but on the Falcon we have to use the
 * ST-DMA chip. There are only multiples of 512 bytes possible and max.
 * 255*512 bytes :-( This means also, that defining READ_OVERRUNS is not
 * possible on the Falcon, since that would require to program the DMA for
 * n*512 - atari_read_overrun bytes. But it seems that the Falcon doesn't have
 * the overrun problem, so this question is academic :-)
 */

static unsigned long atari_dma_xfer_len(unsigned long wanted_len,
					struct scsi_cmnd *cmd, int write_flag)
{
	unsigned long	possible_len, limit;

	if (IS_A_TT())
		/* TT SCSI DMA can transfer arbitrary #bytes */
		return wanted_len;

	/* ST DMA chip is stupid -- only multiples of 512 bytes! (and max.
	 * 255*512 bytes, but this should be enough)
	 *
	 * ++roman: Aaargl! Another Falcon-SCSI problem... There are some commands
	 * that return a number of bytes which cannot be known beforehand. In this
	 * case, the given transfer length is an "allocation length". Now it
	 * can happen that this allocation length is a multiple of 512 bytes and
	 * the DMA is used. But if not n*512 bytes really arrive, some input data
	 * will be lost in the ST-DMA's FIFO :-( Thus, we have to distinguish
	 * between commands that do block transfers and those that do byte
	 * transfers. But this isn't easy... there are lots of vendor specific
	 * commands, and the user can issue any command via the
	 * SCSI_IOCTL_SEND_COMMAND.
	 *
	 * The solution: We classify SCSI commands in 1) surely block-mode cmd.s,
	 * 2) surely byte-mode cmd.s and 3) cmd.s with unknown mode. In case 1)
	 * and 3), the thing to do is obvious: allow any number of blocks via DMA
	 * or none. In case 2), we apply some heuristic: Byte mode is assumed if
	 * the transfer (allocation) length is < 1024, hoping that no cmd. not
	 * explicitly known as byte mode have such big allocation lengths...
	 * BTW, all the discussion above applies only to reads. DMA writes are
	 * unproblematic anyways, since the targets aborts the transfer after
	 * receiving a sufficient number of bytes.
	 *
	 * Another point: If the transfer is from/to an non-ST-RAM address, we
	 * use the dribble buffer and thus can do only STRAM_BUFFER_SIZE bytes.
	 */

	if (write_flag) {
		/* Write operation can always use the DMA, but the transfer size must
		 * be rounded up to the next multiple of 512 (atari_dma_setup() does
		 * this).
		 */
		possible_len = wanted_len;
	} else {
		/* Read operations: if the wanted transfer length is not a multiple of
		 * 512, we cannot use DMA, since the ST-DMA cannot split transfers
		 * (no interrupt on DMA finished!)
		 */
		if (wanted_len & 0x1ff)
			possible_len = 0;
		else {
			/* Now classify the command (see above) and decide whether it is
			 * allowed to do DMA at all */
			switch (falcon_classify_cmd(cmd)) {
			case CMD_SURELY_BLOCK_MODE:
				possible_len = wanted_len;
				break;
			case CMD_SURELY_BYTE_MODE:
				possible_len = 0; /* DMA prohibited */
				break;
			case CMD_MODE_UNKNOWN:
			default:
				/* For unknown commands assume block transfers if the transfer
				 * size/allocation length is >= 1024 */
				possible_len = (wanted_len < 1024) ? 0 : wanted_len;
				break;
			}
		}
	}

	/* Last step: apply the hard limit on DMA transfers */
	limit = (atari_dma_buffer && !STRAM_ADDR(virt_to_phys(cmd->SCp.ptr))) ?
		    STRAM_BUFFER_SIZE : 255*512;
	if (possible_len > limit)
		possible_len = limit;

	if (possible_len != wanted_len)
		dprintk(NDEBUG_DMA, "Sorry, must cut DMA transfer size to %ld bytes "
			   "instead of %ld\n", possible_len, wanted_len);

	return possible_len;
}


#endif	/* REAL_DMA */


/* NCR5380 register access functions
 *
 * There are separate functions for TT and Falcon, because the access
 * methods are quite different. The calling macros NCR5380_read and
 * NCR5380_write call these functions via function pointers.
 */

static unsigned char atari_scsi_tt_reg_read(unsigned char reg)
{
	return tt_scsi_regp[reg * 2];
}

static void atari_scsi_tt_reg_write(unsigned char reg, unsigned char value)
{
	tt_scsi_regp[reg * 2] = value;
}

static unsigned char atari_scsi_falcon_reg_read(unsigned char reg)
{
	dma_wd.dma_mode_status= (u_short)(0x88 + reg);
	return (u_char)dma_wd.fdc_acces_seccount;
}

static void atari_scsi_falcon_reg_write(unsigned char reg, unsigned char value)
{
	dma_wd.dma_mode_status = (u_short)(0x88 + reg);
	dma_wd.fdc_acces_seccount = (u_short)value;
}


#include "atari_NCR5380.c"

static int atari_scsi_bus_reset(struct scsi_cmnd *cmd)
{
	int rv;
	unsigned long flags;

	local_irq_save(flags);

#ifdef REAL_DMA
	/* Abort a maybe active DMA transfer */
	if (IS_A_TT()) {
		tt_scsi_dma.dma_ctrl = 0;
	} else {
		st_dma.dma_mode_status = 0x90;
		atari_dma_active = 0;
		atari_dma_orig_addr = NULL;
	}
#endif

	rv = NCR5380_bus_reset(cmd);

	/* The 5380 raises its IRQ line while _RST is active but the ST DMA
	 * "lock" has been released so this interrupt may end up handled by
	 * floppy or IDE driver (if one of them holds the lock). The NCR5380
	 * interrupt flag has been cleared already.
	 */

	local_irq_restore(flags);

	return rv;
}

#define DRV_MODULE_NAME         "atari_scsi"
#define PFX                     DRV_MODULE_NAME ": "

static struct scsi_host_template atari_scsi_template = {
	.module			= THIS_MODULE,
	.proc_name		= DRV_MODULE_NAME,
	.show_info		= atari_scsi_show_info,
	.name			= "Atari native SCSI",
	.info			= atari_scsi_info,
	.queuecommand		= atari_scsi_queue_command,
	.eh_abort_handler	= atari_scsi_abort,
	.eh_bus_reset_handler	= atari_scsi_bus_reset,
	.this_id		= 7,
	.use_clustering		= DISABLE_CLUSTERING
};

static int __init atari_scsi_probe(struct platform_device *pdev)
{
	struct Scsi_Host *instance;
	int error;
	struct resource *irq;
	int host_flags = 0;

	irq = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
	if (!irq)
		return -ENODEV;

	if (ATARIHW_PRESENT(TT_SCSI)) {
		atari_scsi_reg_read  = atari_scsi_tt_reg_read;
		atari_scsi_reg_write = atari_scsi_tt_reg_write;
	} else {
		atari_scsi_reg_read  = atari_scsi_falcon_reg_read;
		atari_scsi_reg_write = atari_scsi_falcon_reg_write;
	}

	/* The values for CMD_PER_LUN and CAN_QUEUE are somehow arbitrary.
	 * Higher values should work, too; try it!
	 * (But cmd_per_lun costs memory!)
	 *
	 * But there seems to be a bug somewhere that requires CAN_QUEUE to be
	 * 2*CMD_PER_LUN. At least on a TT, no spurious timeouts seen since
	 * changed CMD_PER_LUN...
	 *
	 * Note: The Falcon currently uses 8/1 setting due to unsolved problems
	 * with cmd_per_lun != 1
	 */
	if (ATARIHW_PRESENT(TT_SCSI)) {
		atari_scsi_template.can_queue    = 16;
		atari_scsi_template.cmd_per_lun  = 8;
		atari_scsi_template.sg_tablesize = SG_ALL;
	} else {
		atari_scsi_template.can_queue    = 8;
		atari_scsi_template.cmd_per_lun  = 1;
		atari_scsi_template.sg_tablesize = SG_NONE;
	}

	if (setup_can_queue > 0)
		atari_scsi_template.can_queue = setup_can_queue;

	if (setup_cmd_per_lun > 0)
		atari_scsi_template.cmd_per_lun = setup_cmd_per_lun;

	/* Leave sg_tablesize at 0 on a Falcon! */
	if (ATARIHW_PRESENT(TT_SCSI) && setup_sg_tablesize >= 0)
		atari_scsi_template.sg_tablesize = setup_sg_tablesize;

	if (setup_hostid >= 0) {
		atari_scsi_template.this_id = setup_hostid & 7;
	} else {
		/* Test if a host id is set in the NVRam */
		if (ATARIHW_PRESENT(TT_CLK) && nvram_check_checksum()) {
			unsigned char b = nvram_read_byte(14);

			/* Arbitration enabled? (for TOS)
			 * If yes, use configured host ID
			 */
			if (b & 0x80)
				atari_scsi_template.this_id = b & 7;
		}
	}

#ifdef SUPPORT_TAGS
	if (setup_use_tagged_queuing < 0)
		setup_use_tagged_queuing = 0;
#endif

#ifdef REAL_DMA
	/* If running on a Falcon and if there's TT-Ram (i.e., more than one
	 * memory block, since there's always ST-Ram in a Falcon), then
	 * allocate a STRAM_BUFFER_SIZE byte dribble buffer for transfers
	 * from/to alternative Ram.
	 */
	if (ATARIHW_PRESENT(ST_SCSI) && !ATARIHW_PRESENT(EXTD_DMA) &&
	    m68k_num_memory > 1) {
		atari_dma_buffer = atari_stram_alloc(STRAM_BUFFER_SIZE, "SCSI");
		if (!atari_dma_buffer) {
			pr_err(PFX "can't allocate ST-RAM double buffer\n");
			return -ENOMEM;
		}
		atari_dma_phys_buffer = atari_stram_to_phys(atari_dma_buffer);
		atari_dma_orig_addr = 0;
	}
#endif

	instance = scsi_host_alloc(&atari_scsi_template,
	                           sizeof(struct NCR5380_hostdata));
	if (!instance) {
		error = -ENOMEM;
		goto fail_alloc;
	}
	atari_scsi_host = instance;

#ifdef CONFIG_ATARI_SCSI_RESET_BOOT
	atari_scsi_reset_boot();
#endif

	instance->irq = irq->start;

	host_flags |= IS_A_TT() ? 0 : FLAG_LATE_DMA_SETUP;

	NCR5380_init(instance, host_flags);

	if (IS_A_TT()) {
		error = request_irq(instance->irq, scsi_tt_intr, 0,
		                    "NCR5380", instance);
		if (error) {
			pr_err(PFX "request irq %d failed, aborting\n",
			       instance->irq);
			goto fail_irq;
		}
		tt_mfp.active_edge |= 0x80;	/* SCSI int on L->H */
#ifdef REAL_DMA
		tt_scsi_dma.dma_ctrl = 0;
		atari_dma_residual = 0;

		/* While the read overruns (described by Drew Eckhardt in
		 * NCR5380.c) never happened on TTs, they do in fact on the
		 * Medusa (This was the cause why SCSI didn't work right for
		 * so long there.) Since handling the overruns slows down
		 * a bit, I turned the #ifdef's into a runtime condition.
		 *
		 * In principle it should be sufficient to do max. 1 byte with
		 * PIO, but there is another problem on the Medusa with the DMA
		 * rest data register. So read_overruns is currently set
		 * to 4 to avoid having transfers that aren't a multiple of 4.
		 * If the rest data bug is fixed, this can be lowered to 1.
		 */
		if (MACH_IS_MEDUSA) {
			struct NCR5380_hostdata *hostdata =
				shost_priv(instance);

			hostdata->read_overruns = 4;
		}
#endif
	} else {
		/* Nothing to do for the interrupt: the ST-DMA is initialized
		 * already.
		 */
#ifdef REAL_DMA
		atari_dma_residual = 0;
		atari_dma_active = 0;
		atari_dma_stram_mask = (ATARIHW_PRESENT(EXTD_DMA) ? 0x00000000
					: 0xff000000);
#endif
	}

	error = scsi_add_host(instance, NULL);
	if (error)
		goto fail_host;

	platform_set_drvdata(pdev, instance);

	scsi_scan_host(instance);
	return 0;

fail_host:
	if (IS_A_TT())
		free_irq(instance->irq, instance);
fail_irq:
	NCR5380_exit(instance);
	scsi_host_put(instance);
fail_alloc:
	if (atari_dma_buffer)
		atari_stram_free(atari_dma_buffer);
	return error;
}

static int __exit atari_scsi_remove(struct platform_device *pdev)
{
	struct Scsi_Host *instance = platform_get_drvdata(pdev);

	scsi_remove_host(instance);
	if (IS_A_TT())
		free_irq(instance->irq, instance);
	NCR5380_exit(instance);
	scsi_host_put(instance);
	if (atari_dma_buffer)
		atari_stram_free(atari_dma_buffer);
	return 0;
}

static struct platform_driver atari_scsi_driver = {
	.remove = __exit_p(atari_scsi_remove),
	.driver = {
		.name	= DRV_MODULE_NAME,
		.owner	= THIS_MODULE,
	},
};

module_platform_driver_probe(atari_scsi_driver, atari_scsi_probe);

MODULE_ALIAS("platform:" DRV_MODULE_NAME);
MODULE_LICENSE("GPL");