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authorVishwanath BS <vishwanath.bs@ti.com>2011-01-21 16:24:58 +0530
committerNicolas Pitre <nicolas.pitre@linaro.org>2011-02-21 23:10:45 -0500
commit0b468f7940283738b097bde55f262205e10491c0 (patch)
tree3d780c23e1da6372e5d740a5786d0b0aa82d3f86
parent55bdeeb8ad4ee42ecd7f430b0c85710d4898ce31 (diff)
downloadlinux-linaro-0b468f7940283738b097bde55f262205e10491c0.tar.gz
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 *.