Re: [PATCH 4/8] omap_hsmmc: set DVFS/PM constraints

Paul Walmsley
2010-01-13T22:53:34Z

(added Denis and Kevin)

Hello Denis, Adrian,

On Wed, 13 Jan 2010, Adrian Hunter wrote:

> >From afab8b432b37ae1f42b281e58989c8d607ed7183 Mon Sep 17 00:00:00 2001
> From: Denis Karpov <ext-denis.2.karpov@nokia.com>
> Date: Wed, 8 Jul 2009 16:15:08 +0200
> Subject: [PATCH] omap_hsmmc: set DVFS/PM constraints
>
> Set constraint for MPU minimal frequency to maintain good
> I/O performance.
>
> The constraint is set in MMC host 'ENABLED' state and dropped
> when MMC host enters 'DISABLED' state which currently happens
> upon 100ms of inactivity.
>
> Signed-off-by: Denis Karpov <ext-denis.2.karpov@nokia.com>
> Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
> ---
> arch/arm/mach-omap2/board-rx51-peripherals.c | 18 ++++++++++++++++++
> arch/arm/mach-omap2/hsmmc.c | 2 ++
> arch/arm/mach-omap2/hsmmc.h | 2 ++
> arch/arm/plat-omap/include/plat/mmc.h | 3 +++
> drivers/mmc/host/omap_hsmmc.c | 17 +++++++++++++++++
> 5 files changed, 42 insertions(+), 0 deletions(-)
>
> diff --git a/arch/arm/mach-omap2/board-rx51-peripherals.c b/arch/arm/mach-omap2/board-rx51-peripherals.c
> index ab07ca2..b6318b1 100644
> --- a/arch/arm/mach-omap2/board-rx51-peripherals.c
> +++ b/arch/arm/mach-omap2/board-rx51-peripherals.c
> @@ -209,6 +209,22 @@ static struct twl4030_madc_platform_data rx51_madc_data = {
> .irq_line = 1,
> };
>
> +#if defined(CONFIG_BRIDGE_DVFS)
> +/*
> + * This handler can be used for setting other DVFS/PM constraints:
> + * intr latency, wakeup latency, DMA start latency, bus throughput
> + * according to API in mach/omap-pm.h. Currently we only set constraints
> + * for MPU frequency.
> + */
> +#define MMC_MIN_MPU_FREQUENCY 500000000 /* S500M at OPP3 */
> +static void rx51_mmc_set_pm_constraints(struct device *dev, int on)
> +{
> + omap_pm_set_min_mpu_freq(dev, (on ? MMC_MIN_MPU_FREQUENCY : 0));
> +}

NAK. The MMC driver (or any other driver, for that matter) must not set
MPU frequency constraints merely to boost performance. Drivers have no
way of knowing what the power vs. performance policy is for a given device
or use case.

If the system is not upscaling MPU frequency quickly enough, then the
power management policy code -- CPUFreq, in the MPU case -- or the
parameters for that code -- need to be adjusted. (Of course, integrators
can always dump hacks like this in their own trees if they get lazy, but
these should be kept far, far away from mainline.)

N.B. As a separate matter, the MMC driver should call
omap_pm_set_min_bus_tput() with the maximum throughput that the current
MMC card can sustain to memory. A reasonable upper bound should be easy
to calculate based on the MMC clock speed and the width of the MMC
transfers. This will allow the kernel to adjust the bus frequency
appropriately as the OMAP PM core's bus utilization model improves.

- Paul