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Diffstat (limited to 'Documentation/arm/Sharp-LH/IOBarrier')
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diff --git a/Documentation/arm/Sharp-LH/IOBarrier b/Documentation/arm/Sharp-LH/IOBarrier deleted file mode 100644 index 2e953e228f4..00000000000 --- a/Documentation/arm/Sharp-LH/IOBarrier +++ /dev/null @@ -1,45 +0,0 @@ -README on the IOBARRIER for CardEngine IO -========================================= - -Due to an unfortunate oversight when the Card Engines were designed, -the signals that control access to some peripherals, most notably the -SMC91C9111 ethernet controller, are not properly handled. - -The symptom is that some back to back IO with the peripheral returns -unreliable data. With the SMC chip, you'll see errors about the bank -register being 'screwed'. - -The cause is that the AEN signal to the SMC chip does not transition -for every memory access. It is driven through the CPLD from the CS7 -line of the CPU's static memory controller which is optimized to -eliminate unnecessary transitions. Yet, the SMC requires a transition -for every write access. The Sharp website has more information about -the effect this power-conserving feature has on peripheral -interfacing. - -The solution is to follow every write access to the SMC chip with an -access to another memory region that will force the CPU to release the -chip select line. It is important to guarantee that this access -forces the CPU off-chip. We map a page of SDRAM as if it were an -uncacheable IO device and read from it after every SMC IO write -operation. - - SMC IO - BARRIER IO - -Only this sequence is important. It does not matter that there is no -BARRIER IO before the access to the SMC chip because the AEN latch -only needs occurs after the SMC IO write cycle. The routines that -implement this work-around make an additional concession which is to -disable interrupts during the IO sequence. Other hardware devices -(the LogicPD CPLD) have registers in the same physical memory -region as the SMC chip. An interrupt might allow an access to one of -those registers while SMC IO is being performed. - -You might be tempted to think that we have to access another device -attached to the static memory controller, but the empirical evidence -indicates that this is not so. Mapping 0x00000000 (flash) and -0xc0000000 (SDRAM) appear to have the same effect. Using SDRAM seems -to be faster. Choosing to access an undecoded memory region is not -desirable as there is no way to know how that chip select will be used -in the future. |