On Wed, Oct 19, 2011 at 4:48 PM, Sujit Reddy Thumma
<sthumma@xxxxxxxxxxxxxx> wrote:
> Current clock gating framework disables the MCI clock as soon as the
> request is completed and enables it when a request arrives. This aggressive
> clock gating framework when enabled cause following issues:
>
> When there are back-to-back requests from the Queue layer, we unnecessarily
> end up disabling and enabling the clocks between these requests since 8 MCLK
> clock cycles is a very short duration compared to the time delay between
> back to back requests reaching the MMC layer. This overhead can effect the
> overall performance depending on how long the clock enable and disable
> calls take which is platform dependent. For example on some platforms we
> can have clock control not on the local processor, but on a different
> subsystem and the time taken to perform the clock enable/disable can add
> significant overhead.
OK I see the problem. At one point it was suggested to use the delayed
disable facilities in runtime PM for avoiding this, but it never materialized.
> Fix this by delaying turning off the clocks by posting request on
> delayed workqueue. Also cancel the unscheduled pending work, if any,
> when there is access to card.
(...)
> @@ -252,10 +252,11 @@ struct mmc_host {
> int clk_requests; /* internal reference counter */
> unsigned int clk_delay; /* number of MCI clk hold cycles */
> bool clk_gated; /* clock gated */
> - struct work_struct clk_gate_work; /* delayed clock gate */
> + struct delayed_work clk_gate_work; /* delayed clock gate */
> unsigned int clk_old; /* old clock value cache */
> spinlock_t clk_lock; /* lock for clk fields */
> struct mutex clk_gate_mutex; /* mutex for clock gating */
> +#define MMC_CLK_GATE_DELAY 50 /* in milliseconds*/
> #endif
What's the rationale of having this in the middle of the struct
in the .h-file?
Move it inside the #ifdef in host.c instead, and also provide
some rationale about the choice of 50 ms.
Maybe we should even provide a sysfs or debugfs entry for
tweaking that value, as has been suggested in the past.
However that's no big deal for me...
Do you have a patch to your msm_sdcc.c that enables the
gating to take effect as well? Does it solve the problems
pointed out by Russell when I tried the same type of patch
to mmci.c?
http://marc.info/?l=linux-mmc&m=129146545916794&w=2
Yours,
Linus Walleij
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