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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 EST.

1	
2	The OMAP PM interface
3	=====================
4	
5	This document describes the temporary OMAP PM interface.  Driver
6	authors use these functions to communicate minimum latency or
7	throughput constraints to the kernel power management code.
8	Over time, the intention is to merge features from the OMAP PM
9	interface into the Linux PM QoS code.
10	
11	Drivers need to express PM parameters which:
12	
13	- support the range of power management parameters present in the TI SRF;
14	
15	- separate the drivers from the underlying PM parameter
16	  implementation, whether it is the TI SRF or Linux PM QoS or Linux
17	  latency framework or something else;
18	
19	- specify PM parameters in terms of fundamental units, such as
20	  latency and throughput, rather than units which are specific to OMAP
21	  or to particular OMAP variants;
22	
23	- allow drivers which are shared with other architectures (e.g.,
24	  DaVinci) to add these constraints in a way which won't affect non-OMAP
25	  systems,
26	
27	- can be implemented immediately with minimal disruption of other
28	  architectures.
29	
30	
31	This document proposes the OMAP PM interface, including the following
32	five power management functions for driver code:
33	
34	1. Set the maximum MPU wakeup latency:
35	   (*pdata->set_max_mpu_wakeup_lat)(struct device *dev, unsigned long t)
36	
37	2. Set the maximum device wakeup latency:
38	   (*pdata->set_max_dev_wakeup_lat)(struct device *dev, unsigned long t)
39	
40	3. Set the maximum system DMA transfer start latency (CORE pwrdm):
41	   (*pdata->set_max_sdma_lat)(struct device *dev, long t)
42	
43	4. Set the minimum bus throughput needed by a device:
44	   (*pdata->set_min_bus_tput)(struct device *dev, u8 agent_id, unsigned long r)
45	
46	5. Return the number of times the device has lost context
47	   (*pdata->get_dev_context_loss_count)(struct device *dev)
48	
49	
50	Further documentation for all OMAP PM interface functions can be
51	found in arch/arm/plat-omap/include/mach/omap-pm.h.
52	
53	
54	The OMAP PM layer is intended to be temporary
55	---------------------------------------------
56	
57	The intention is that eventually the Linux PM QoS layer should support
58	the range of power management features present in OMAP3.  As this
59	happens, existing drivers using the OMAP PM interface can be modified
60	to use the Linux PM QoS code; and the OMAP PM interface can disappear.
61	
62	
63	Driver usage of the OMAP PM functions
64	-------------------------------------
65	
66	As the 'pdata' in the above examples indicates, these functions are
67	exposed to drivers through function pointers in driver .platform_data
68	structures.  The function pointers are initialized by the board-*.c
69	files to point to the corresponding OMAP PM functions:
70	.set_max_dev_wakeup_lat will point to
71	omap_pm_set_max_dev_wakeup_lat(), etc.  Other architectures which do
72	not support these functions should leave these function pointers set
73	to NULL.  Drivers should use the following idiom:
74	
75	        if (pdata->set_max_dev_wakeup_lat)
76	            (*pdata->set_max_dev_wakeup_lat)(dev, t);
77	
78	The most common usage of these functions will probably be to specify
79	the maximum time from when an interrupt occurs, to when the device
80	becomes accessible.  To accomplish this, driver writers should use the
81	set_max_mpu_wakeup_lat() function to constrain the MPU wakeup
82	latency, and the set_max_dev_wakeup_lat() function to constrain the
83	device wakeup latency (from clk_enable() to accessibility).  For
84	example,
85	
86	        /* Limit MPU wakeup latency */
87	        if (pdata->set_max_mpu_wakeup_lat)
88	            (*pdata->set_max_mpu_wakeup_lat)(dev, tc);
89	
90	        /* Limit device powerdomain wakeup latency */
91	        if (pdata->set_max_dev_wakeup_lat)
92	            (*pdata->set_max_dev_wakeup_lat)(dev, td);
93	
94	        /* total wakeup latency in this example: (tc + td) */
95	
96	The PM parameters can be overwritten by calling the function again
97	with the new value.  The settings can be removed by calling the
98	function with a t argument of -1 (except in the case of
99	set_max_bus_tput(), which should be called with an r argument of 0).
100	
101	The fifth function above, omap_pm_get_dev_context_loss_count(),
102	is intended as an optimization to allow drivers to determine whether the
103	device has lost its internal context.  If context has been lost, the
104	driver must restore its internal context before proceeding.
105	
106	
107	Other specialized interface functions
108	-------------------------------------
109	
110	The five functions listed above are intended to be usable by any
111	device driver.  DSPBridge and CPUFreq have a few special requirements.
112	DSPBridge expresses target DSP performance levels in terms of OPP IDs.
113	CPUFreq expresses target MPU performance levels in terms of MPU
114	frequency.  The OMAP PM interface contains functions for these
115	specialized cases to convert that input information (OPPs/MPU
116	frequency) into the form that the underlying power management
117	implementation needs:
118	
119	6. (*pdata->dsp_get_opp_table)(void)
120	
121	7. (*pdata->dsp_set_min_opp)(u8 opp_id)
122	
123	8. (*pdata->dsp_get_opp)(void)
124	
125	9. (*pdata->cpu_get_freq_table)(void)
126	
127	10. (*pdata->cpu_set_freq)(unsigned long f)
128	
129	11. (*pdata->cpu_get_freq)(void)
130	
131	Customizing OPP for platform
132	============================
133	Defining CONFIG_PM should enable OPP layer for the silicon
134	and the registration of OPP table should take place automatically.
135	However, in special cases, the default OPP table may need to be
136	tweaked, for e.g.:
137	 * enable default OPPs which are disabled by default, but which
138	   could be enabled on a platform
139	 * Disable an unsupported OPP on the platform
140	 * Define and add a custom opp table entry
141	in these cases, the board file needs to do additional steps as follows:
142	arch/arm/mach-omapx/board-xyz.c
143		#include "pm.h"
144		....
145		static void __init omap_xyz_init_irq(void)
146		{
147			....
148			/* Initialize the default table */
149			omapx_opp_init();
150			/* Do customization to the defaults */
151			....
152		}
153	NOTE: omapx_opp_init will be omap3_opp_init or as required
154	based on the omap family.
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