[PATCH] reset: Add reset controller API

[Amjad Ouled-Ameur]

Hi Philipp,

Thank you very much for the review.

Please find my comments below:

On 02/10/2020 13:14, Philipp Zabel wrote:

> Hi Amjad,
>
> Thank you for the patch, comments below:
>
> On Thu, 2020-10-01 at 15:55 +0200, Amjad Ouled-Ameur wrote:
>> An update on the patch title, since we don't add an API but extend it,
>> The title should rather be: Add a new call to the reset framework
> I think it should even say what functionality is added, for example
>
> "reset: make shared pulsed reset controls re-triggerable"

Will do !

>> Le jeu. 1 oct. 2020 à 15:28, Amjad Ouled-Ameur
>> <aouledameur at baylibre.com> a écrit :
>>> The current reset framework API does not allow to release what is done by
>>> reset_control_reset(), IOW decrement triggered_count. Add the new
>>> reset_control_resettable() call to do so.
>>>
>>> When reset_control_reset() has been called once, the counter
>>> triggered_count, in the reset framework, is incremented i.e the resource
>>> under the reset is in-use and the reset should not be done again.
>>> reset_control_resettable() would be the way to state that the resource is
>>> no longer used and, that from the caller's perspective, the reset can be
>>> fired again if necessary.
>>>
>>> This patch will fix a usb suspend warning seen on the libretech-cc
>>> related to the shared reset line. This warning was addressed by the
>>> previously reverted commit 7a410953d1fb ("usb: dwc3: meson-g12a: fix shared
>>> reset control use")
>>>
>>> Signed-off-by: Amjad Ouled-Ameur <aouledameur at baylibre.com>
>>> Reported-by: Jerome Brunet <jbrunet at baylibre.com>
>>> ---
>>>   drivers/reset/core.c  | 57 +++++++++++++++++++++++++++++++++++++++++++
>>>   include/linux/reset.h |  1 +
>>>   2 files changed, 58 insertions(+)
>>>
>>> diff --git a/drivers/reset/core.c b/drivers/reset/core.c
>>> index 01c0c7aa835c..53653d4b55c4 100644
>>> --- a/drivers/reset/core.c
>>> +++ b/drivers/reset/core.c
>>> @@ -207,6 +207,19 @@ static int reset_control_array_reset(struct reset_control_array *resets)
>>>          return 0;
>>>   }
>>>
>>> +static int reset_control_array_resettable(struct reset_control_array *resets)
>>> +{
>>> +       int ret, i;
>>> +
>>> +       for (i = 0; i < resets->num_rstcs; i++) {
>>> +               ret = reset_control_resettable(resets->rstc[i]);
>>> +               if (ret)
>>> +                       return ret;
>>> +       }
> This is tricky, as we can't really roll back decrementing
> triggered_count in case just one of those fails.
>
> I think reset_control_array_resettable has to be open coded to first
> check for errors and only then loop through all controls and decrement
> their triggered_count.

I agree with this, it is risky to start decrementing before checking for
errors. The V2 will include an open coded version of this function.

>>> +
>>> +       return 0;
>>> +}
>>> +
>>>   static int reset_control_array_assert(struct reset_control_array *resets)
>>>   {
>>>          int ret, i;
>>> @@ -324,6 +337,50 @@ int reset_control_reset(struct reset_control *rstc)
>>>   }
>>>   EXPORT_SYMBOL_GPL(reset_control_reset);
>>>
>>> +/**
>>> + * reset_control_resettable - decrements triggered_count of the controlled device
>>> + * @rstc: reset controller
> It is more important to document the purpose of the function than the
> mechanism by which it is achieved. triggered_count is an implementation
> detail.
>
> Maybe "allow shared reset line to be triggered again" or similar.

Roger that, will do in V2.

>
>>> + *
>>> + * On a shared reset line the actual reset pulse is only triggered once for the
>>> + * lifetime of the reset_control instance, except if this function is used.
>>> + * In fact, It decrements triggered_count that makes sure of not allowing
>>> + * a reset if triggered_count is not null.
>>> + *
>>> + * This is a no-op in case triggered_count is already null i.e shared reset line
>>> + * is ready to be triggered.
> This is not a good idea IMHO. It would be better to document that calls
> to this function must be balanced with calls to reset_control_reset, and
> then throw a big warning below in case deassert_count ever dips below 0.
>
> Otherwise nothing stops drivers from silently decrementing other
> driver's trigger count by accidentally calling this multiple times.

I do agree, accidental calls should be reported by warnings.

>>> + *
>>> + * Consumers must not use reset_control_(de)assert on shared reset lines when
>>> + * reset_control_reset has been used.
>>> + *
>>> + * If rstc is NULL it is an optional clear and the function will just
>>> + * return 0.
>>> + */
>>> +int reset_control_resettable(struct reset_control *rstc)
>>> +{
>>> +       if (!rstc)
>>> +               return 0;
>>> +
>>> +       if (WARN_ON(IS_ERR(rstc)))
>>> +               return -EINVAL;
>>> +
>>> +       if (reset_control_is_array(rstc))
>>> +               return reset_control_array_resettable(rstc_to_array(rstc));
>>> +
>>> +       if (rstc->shared) {
>>> +               if (WARN_ON(atomic_read(&rstc->deassert_count) != 0))
>>> +                       return -EINVAL;
>>> +
>>> +               if (atomic_read(&rstc->triggered_count) > 0)
>>> +                       atomic_dec(&rstc->triggered_count);
> I think this should be
>
> 		WARN_ON(atomic_dec_return(&rstc->triggered_count) < 0);

That is even better, having this warning means that the call has not

been properly used.

>
> regards
> Philipp

Next version of the patch will be sent soon, include everything we

have discussed.

Sincerely,

Amjad