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@xxxxxxxxx> Cc: Tomi Valkeinen <tomi.valkeinen@xxxxxx> Cc: Tero Kristo <tero.kristo@xxxxxxxxx> Cc: Vaibhav Hiremath <hvaibhav@xxxxxx> Cc: Senthilvadivu Guruswamy <svadivu@xxxxxx> Cc: Sumit Semwal <sumit.semwal@xxxxxx> Cc: Jean Pihet <j-pihet@xxxxxx> --- 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) -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
