Re: [PATCH] drm/bridge: ite-it66121: fix silent probe regression due to reset timing

[Luca Ceresoli]

Hello Javier,

On Thu Oct 15, 2026 at 10:28 AM CEST, Javier Martinez Canillas wrote:
> Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx> writes:
>
> Hello Luca,
>
>> Commit e02b5262fd28 ("drm/bridge: it66121: acquire reset GPIO in probe")
>> rightfully added code to make the reset pin really work instead of being a
>> no-op, but that change introduced a regression making probe fail most of
>> the times, at least on some hardware.
>>
>> On a BeagleBone Black rev.D the IT66122 silently fails probing >90% of the
>> times. This is because:
>>
>>  1. the regmap_read() calls just after it66121_hw_reset() to read the ID
>>     registers fail
>>  2. there is no error checking for these regmap_read(), execution
>>     continues silently
>>  3. this results in not matching any of the IDs from it66xx_chip_info
>>  4. and this results in silently returning -ENODEV
>>
>> Empirical tests showed a small delay after de-asserting reset fixes
>> probing. Likely the device needs some ramp-up time after reset. No such
>> requirement is mentioned in the docs I could find, so the amount of delay
>> was chosen by testing: 10 ms worked in 100% of 25 test runs, so setting a
>> 20 ms delay should provide a safe margin.
>>
>> Fixes: e02b5262fd28 ("drm/bridge: it66121: acquire reset GPIO in probe")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
>> ---
>> FYI For items 2 and 4 above, I'm sending separate patches to make those
>> errors not silent. Those aren't fixes, so they would be for drm-misc-next,
>> while this patch is for drm-misc-fixes.
>> ---
>>  drivers/gpu/drm/bridge/ite-it66121.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/gpu/drm/bridge/ite-it66121.c b/drivers/gpu/drm/bridge/ite-it66121.c
>> index 51fc68ffdf8f..692a85e1e2e8 100644
>> --- a/drivers/gpu/drm/bridge/ite-it66121.c
>> +++ b/drivers/gpu/drm/bridge/ite-it66121.c
>> @@ -356,6 +356,7 @@ static void it66121_hw_reset(struct it66121_ctx *ctx)
>>  	gpiod_set_value(ctx->gpio_reset, 1);
>>  	msleep(20);
>>  	gpiod_set_value(ctx->gpio_reset, 0);
>> +	msleep(20);
>
> Maybe adding a comment here mentioning that this value was found by
> testing and is not mentioned in the IT66122 documentation ?

Sure, I'll add in v2:

  /* Post-reset delay not specified, value found by testing */

> In any case, the patch makes sense to me:
>
> Reviewed-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>

Thanks for reviewing!

I assume I can keep your R-by with the above comment, let me know if that's
not the case.

Kind regards,
Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com