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

[Javier Martinez Canillas]

"Luca Ceresoli" <luca.ceresoli@xxxxxxxxxxx> writes:

> 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!
>

You are welcome.

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

That's correct, yes.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat