OMAP: Add DVFS Documentation

Add Documentation for DVFS Framework

Signed-off-by: Vishwanath BS <vishwanath.bs@ti.com>

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+*=============*
+* DVFS Framework *
+*=============*
+(C) 2011 Vishwnath BS <vishwanath.bs@ti.com>, Texas Instruments Incorporated
+Contents
+--------
+1. Introduction
+2. Data Structure Organization
+3. DVFS APIs
+
+1. Introduction
+===============
+DVFS is a technique that uses the optimal operating frequency and voltage to
+allow a task to be performed in the required amount of time.
+OMAP processors have voltage domains whose voltage can be scaled to
+various levels depending on which the operating frequencies of certain
+devices belonging to the domain will also need to be scaled. This voltage
+frequency tuple is known as Operating Performance Point (OPP). A device
+can have multiple OPP's. Also a voltage domain could be shared between
+multiple devices. Also there could be dependencies between various
+voltage domains for maintaining system performance like VDD<X>
+should be at voltage v1 when VDD<Y> is at voltage v2.
+
+The design of this framework takes into account all the above mentioned points.
+To summarize the basic design of DVFS framework:-
+
+1. Have device opp tables for each device whose operating frequency can be
+   scaled. This is easy now due to the existance of hwmod layer which
+   allow storing of device specific info. The device opp tables contain
+   the opp pairs (frequency voltage tuples), the voltage domain pointer
+   to which the device belongs to, the device specific set_rate and
+   get_rate API's which will do the actual scaling of the device frequency
+   and retrieve the current device frequency.
+2. Introduce use counting on a per VDD basis. This is to take care multiple
+   requests to scale a VDD. The VDD will be scaled to the maximum of the
+   voltages requested.
+3. Keep track of all scalable devices belonging to a particular voltage
+   domain the voltage layer.
+4. Keep track of frequency requests for each of the device. This will enable
+   to scale individual devices to different frequency (even w/o scaling voltage
+   aka frequency throttling)
+5. Generic dvfs API that can be called by anybody to scale a device opp.
+   This API takes the device pointer and frequency to which the device
+   needs to be scaled to. This API then internally finds out the voltage
+   domain to which the device belongs to and the voltage to which the voltage
+   domain needs to be put to for the device to be scaled to the new frequency
+   from he device opp table. Then this API will add requested frequency into
+   the corresponding target device frequency list and add voltage request to
+   the corresponding vdd. Subsequently it calls voltage scale function which
+   will find out the highest requested voltage for the given vdd and scales
+   the voltage to the required one. It also runs through the list of all
+   scalable devices belonging to this voltage domain and scale them to the
+   appropriate frequencies using the set_rate pointer in the device opp tables.
+6. Handle inter VDD dependecies.
+
+
+2.  The Core DVFS data structure:
+=================================
+
+					|-------------------|			|-------------------|
+					|User2 (dev2, freq2)|			|User4 (dev4, freq4)|
+					|-------^-----------|			|-------^-----------|
+							|								|
+					|-------|-----------|			|-------|-----------|
+					|User1 (dev1, freq1)|			|User3 (dev3, freq3)|(omap_dev_user_list)
+					|-------^-----------|			|-------^-----------|
+                            |                               |
+						|---|--------------|             |------------------|
+		     |--------->| DEV1 (dev)       |------------>| DEV2 (dev)       |(omap_vdd_dev_list)
+			 |			|omap_dev_user_list|			 |omap_dev_user_list|
+			 |			|------------------|             |------------------|
+			 |
+   |---------|-----------|
+   |       VDD_n         |
+   |  omap_vdd_dev_list  |
+   |  omap_vdd_user_list |(omap_vdd_dvfs_info)
+   |                     |
+   |--------|------------|
+			|
+            |           |------------|  |------------|  |--------------|
+            |---------> | vdd_user1  |->|  vdd_user2 |->|   vdd_user3  | (omap_vdd_user_list)
+						| (dev, volt)|	| (dev, volt)|  | (dev, volt)  |
+						|------------|	|------------|	|--------------|
+
+3.  APIs:
+ =====
+ 1. omap_device_scale - Set a new rate at which the device is to operate
+
+  Examples:
+  1. Simple Device scaling:
+  Suppose module M wants to put device dev1 to frequency f1. Let's say that mdev
+  is the device * for module M. Then this could be achieved by
+  ret = omap_device_scale(mdev, dev1, f1)
+  if (ret)
+	/* handle error *.
+
+  2. Frequency Throttling
+  Suppose say there are 2 modules M1 and M2 in Voltage domain VDDx.
+  Module M1 wants to set VDDx to OPP100 voltage which means M1 and M2 will
+  be running at OPP100 frequency. Suppose Module M2 wants to run at OPP50
+  frequency (say f2_opp50) instead of OPP100. This can be achieved by
+
+  /* this operation will place M1 and M2 to run at OPP100 */
+  ret = omap_device_scale(mdev1, dev1, f1_opp100);
+  if (ret)
+	/* handle error *.
+
+  /* This operation will bring M2 to run at f2_opp50 w/o decreasing VDDx voltage */
+  ret = omap_device_scale(mdev2, dev2, f2_opp50);
+  if (ret)
+	/* handle error *.