[PATCH] ARM: dt: bcm2835: set mmc clock to the correct value of 250MHz

Eric Anholt eric at anholt.net
Tue Sep 8 16:13:41 PDT 2015

kernel at martin.sperl.org writes:

> From: Martin Sperl <kernel at martin.sperl.org>
> This patch sets the clock speed to 250MHz to make the SD card
> work at the correct clock speed without overclocking by 2.5.
> Signed-off-by: Martin Sperl <kernel at martin.sperl.org>
> ---
>  arch/arm/boot/dts/bcm2835.dtsi |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> Analysis with a logic-analyzer found that the current setting
> of the mmc clock at 100MHz is wrong and results in overclocking
> the SD-card and eMMC on the Compute Module.
> Exact measurements show that when 25Mhz are selected by the sdhci
> driver effectively 62.5MHz are used (so 2.5 times faster), which is
> what is expected when the clock is configured to a fixed 100MHz
> instead of the 250MHz that the clock is actually running at.

Looks like this became the default in the firmware in November 2013 (you
can, of course, still do lots of configuration that would change it
higher or lower).

I don't know what the downside would be of running MMC with too large of
a divisor for devices running pre-November firmware.  I suspect we
should just wait until the new clock driver lands, to avoid any
backwards-compatibility issues.  Sorry for the trouble :(

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