Re: How to prevent high speed mode due to signal integrity problem

[Caleb Crome <caleb@xxxxxxxxx>]

On Mon, Dec 5, 2016 at 4:46 PM, Fabio Estevam <festevam@xxxxxxxxx> wrote:
> Hi Caleb,
>
> On Mon, Dec 5, 2016 at 10:36 PM, Caleb Crome <caleb@xxxxxxxxx> wrote:
>> Hello,
>>    We have a board here where there is a signal integrity problem on the MMC
>> bus.  It works fine at 25MHz, but does not work in higher speed modes.
>>
>> Is there an easy way to disable all the high speed modes?  Perhaps from the
>> device tree?
>>
>> I tried searching the archives, but gmane seems to be not working.
>>
>> I'm using a stock 4.8.11 kernel.
>> Host processor is i.MX6
>
> Would max-frequency = <25000000>; do the trick?
>
> Please take a look at Documentation/devicetree/bindings/mmc/mmc.txt .

Hi Fabio,
  Thanks for the quick feedback -- I was momentarily hopeful...
Unfortunatley, that doesn't seem to work.

I have set (using a solid-run hummingboard uSOM):
&usdhc2 {
    pinctrl-names = "default";
    pinctrl-0 = <
    &pinctrl_hummingboard_usdhc2_aux
   &pinctrl_hummingboard_usdhc2
   >;
   vmmc-supply = <&reg_3p3v>;
   cd-gpios = <&gpio1 4 GPIO_ACTIVE_LOW>;
   max-frequency = <25000000>;
   status = "okay";
};

and just for good measure I set:
&usdhc1 {
    max-frequency = <25000000>;
};
&usdhc3 {
   max-frequency = <25000000>;
};
&usdhc4 {
   max-frequency = <25000000>;
};
(though I'm pretty sure it's using SD2).

And it not only attempts to clock at 50MHz, but also at 200 MHz -- way
beyond what the board can support.

Basically, it doesn't seem to be heeding the max-frequency DT setting at all.

I did in fact verify that the max-frequency value was read at
http://lxr.free-electrons.com/source/drivers/mmc/core/host.c#L216, and
the host->f_max frequency was set to ‭25000000‬, but that f_max was
ignored it seems.

Any other ideas?
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