[PATCH 2/2] OMAP3: cpuidle: add more details to the DSS-related CORE power domain state restriction
Paul Walmsley
paul at pwsan.com
Tue Mar 1 17:49:25 EST 2011
Provide some more details about the rationale behind the CORE power domain
state restriction while DSS is enabled -- hopefully, enough to enable others
to implement an approach based on the DSS's required wakeup latency,
which can vary based on quite a few parameters.
Signed-off-by: Paul Walmsley <paul at pwsan.com>
Cc: Tomi Valkeinen <tomi.valkeinen at ti.com>
Cc: Tero Kristo <tero.kristo at nokia.com>
Cc: Vaibhav Hiremath <hvaibhav at ti.com>
Cc: Senthilvadivu Guruswamy <svadivu at ti.com>
Cc: Sumit Semwal <sumit.semwal at ti.com>
Cc: Jean Pihet <j-pihet at ti.com>
---
arch/arm/mach-omap2/cpuidle34xx.c | 28 +++++++++++++++++++++++++++-
1 files changed, 27 insertions(+), 1 deletions(-)
diff --git a/arch/arm/mach-omap2/cpuidle34xx.c b/arch/arm/mach-omap2/cpuidle34xx.c
index d1b7789..6bdbb33 100644
--- a/arch/arm/mach-omap2/cpuidle34xx.c
+++ b/arch/arm/mach-omap2/cpuidle34xx.c
@@ -269,7 +269,33 @@ static int omap3_enter_idle_bm(struct cpuidle_device *dev,
goto select_state;
}
- /* If DSS is active, prevent CORE RET/OFF */
+ /*
+ * If DSS is active, prevent any CORE power domain transition
+ * to RETENTION or lower. This is because we don't currently
+ * have the infrastructure in the DSS driver or kernel to
+ * enter CORE RET without causing DSS FIFO underflows. When
+ * the DSS FIFO underflows, the screen image can be distorted,
+ * because the CORE may not be able to come back on-line
+ * quickly enough to service DSS requests to the SDRAM
+ * framebuffer. The DSS may also interrupt the MPU when the
+ * FIFO underflows, potentially wasting power.
+ *
+ * The approach below is easy to implement, but potentially
+ * wastes power, especially with screens that don't require a
+ * backlight. Probably the best long-term way to fix this is
+ * for the DSS driver to constrain the CORE and SDRAM minimum
+ * power state with omap_pm_set_max_dev_wakeup_lat(), based on
+ * the DSS FIFO size(s), watermark levels, drain rate, and
+ * refill time. Based on that constraint, the OMAP PM code
+ * should adjust the CORE maximum power state, CORE DPLL
+ * autoidle mode, CORE_CLK rate, and voltage scaling to ensure
+ * the required wakeup latency at the lowest power
+ * consumption.
+ *
+ * (More information on the DSS low-power refresh mode can be
+ * found in _Using Display Low-Power Refresh on the OMAP3430
+ * Device_, TI Application Report, SWPA158 - October 2008.)
+ */
dss_state = pwrdm_read_pwrst(dss_pd);
if (dss_state == PWRDM_POWER_ON &&
core_next_state != PWRDM_POWER_ON)
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