Re: [PATCH v2] backlight: pwm: reject legacy pwm request for device defined in dt

[Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>]

On 12.10.2015 17:06, Boris Brezillon wrote:
> On Mon, 12 Oct 2015 16:54:39 +0300
> Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com> wrote:
> 
>> Hi Boris,
>>
>> On 12.10.2015 16:30, Boris Brezillon wrote:
>>> Hi Vladimir,
>>>
>>> On Mon, 12 Oct 2015 15:16:44 +0200
>>> Nicolas Ferre <nicolas.ferre@atmel.com> wrote:
>>>
>>>> Le 12/10/2015 14:29, Vladimir Zapolskiy a écrit :
>>>>> Platform PWM backlight data provided by board's device tree should be
>>>>> complete enough to successfully request a pwm device using pwm_get()
>>>>> API. This change fixes a bug, when an arbitrary (first found) PWM is
>>>>> connected to a "pwm-backlight" compatible device, when explicit PWM
>>>>> device reference is not given.
>>>>>
>>>>> Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
>>>>> already describes "pwms" as a required property, instead of blind
>>>>> selection of a potentially wrong PWM reject legacy PWM device
>>>>> registration request, leave legacy API only for non-dt cases.
>>>>>
>>>>> Based on initial implementation done by Dmitry Eremin-Solenikov.
>>>>>
>>>>> Reported-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
>>>>> Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
>>>>> Acked-by: Thierry Reding <thierry.reding@gmail.com>
>>>>> Acked-by: Lee Jones <lee.jones@linaro.org>
>>>>
>>>> It seems good to me:
>>>> Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
>>>>
>>>> (Adding some people to the Cc: list).
>>>>
>>>>
>>>>> ---
>>>>> The change is based on lee-backlight/for-backlight-next
>>>>>
>>>>> Changes from v1 to v2:
>>>>> * rebased on top of Nicolas' commit
>>>>>     68feaca0b13 ("backlight: pwm: Handle EPROBE_DEFER while requesting the PWM")
>>>>>
>>>>> Links to previous discussions of the change:
>>>>> * https://patchwork.ozlabs.org/patch/483993/
>>>>> * https://patchwork.ozlabs.org/patch/398849/
>>>>>
>>>>>  drivers/video/backlight/pwm_bl.c | 19 +++++++++----------
>>>>>  1 file changed, 9 insertions(+), 10 deletions(-)
>>>>>
>>>>> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
>>>>> index eff379b..ae3c6b6 100644
>>>>> --- a/drivers/video/backlight/pwm_bl.c
>>>>> +++ b/drivers/video/backlight/pwm_bl.c
>>>>> @@ -271,19 +271,18 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>>>>>  	}
>>>>>  
>>>>>  	pb->pwm = devm_pwm_get(&pdev->dev, NULL);
>>>>> -	if (IS_ERR(pb->pwm)) {
>>>>> -		ret = PTR_ERR(pb->pwm);
>>>>> -		if (ret = -EPROBE_DEFER)
>>>>> -			goto err_alloc;
>>>>> -
>>>>> +	if (IS_ERR(pb->pwm) && PTR_ERR(pb->pwm) != -EPROBE_DEFER
>>>>> +	    && !pdev->dev.of_node) {
>>>>>  		dev_err(&pdev->dev, "unable to request PWM, trying legacy API\n");
>>>>>  		pb->legacy = true;
>>>>>  		pb->pwm = pwm_request(data->pwm_id, "pwm-backlight");
>>>>> -		if (IS_ERR(pb->pwm)) {
>>>>> -			dev_err(&pdev->dev, "unable to request legacy PWM\n");
>>>>> -			ret = PTR_ERR(pb->pwm);
>>>>> -			goto err_alloc;
>>>>> -		}
>>>>> +	}
>>>>> +
>>>>> +	if (IS_ERR(pb->pwm)) {
>>>>> +		ret = PTR_ERR(pb->pwm);
>>>>> +		if (ret != -EPROBE_DEFER)
>>>>> +			dev_err(&pdev->dev, "unable to request PWM\n");
>>>>> +		goto err_alloc;
>>>>>  	}
>>>>>  
>>>>>  	dev_dbg(&pdev->dev, "got pwm for backlight\n");
>>>>>
>>>>
>>>>
>>>
>>> I still think it would be cleaner to do what Thierry proposed here [1].
>>> IMO, embedding the complexity of different error cases depending on the
>>> way PWM devices were defined (OF, pdata, ...) is rather risky and
>>> make the code even more complicated.
>>
>> please correct me if I'm wrong, I suppose Thierry's change fixes
>> Nicolas' commit 68feaca0b13 only, and the intention of my change is to
>> fix an absolutely unrelated problem, see the commit message.
>>
>> So, since still there is a remained chance of getting -EPROBE_DEFER from
>> pwm_get(), e.g. from of_pwm_get() or failed pwmchip_find_by_name() or
>> pwm->chip->ops->request() I don't see how Thierry's change alone may
>> help me to overcome the problem I'm trying to solve here.
> 
> The only valid case where EPROBE_DEFER should be returned is when we
> have a device that is not ready to be used yet (but we're sure that we
> have this device declared, using either the PWM lookup table or the DT
> definition in the PWM subsystem case).

That's fine, and it is reflected in my change.

> Thierry's patch makes sure that EPROBE_DEFER is not returned when the
> PWM device definition is not found using in the PWM lookup tables or
> the DT definition,

This is okay, but I'm interested in proper handling of cases other than
EPROBE_DEFER. EPROBE_DEFER and the related issues are on your balance
and I'm attempting to avoid interfering with it here :)

> and in this case the pwm_bl code will fallback to
> the legacy PWM API, which AFAICT is what you're trying to solve.

Fallback must happen exclusively under (IS_ERR(pb->pwm) &&
PTR_ERR(pb->pwm) != -EPROBE_DEFER && !pdev->dev.of_node) condition IMHO.

Before EPROBE_DEFER appeared on the scene the condition was
(IS_ERR(pb->pwm) && !pdev->dev.of_node).

So, the question is if my change requires any updates or not from your
point of view.

--
With best wishes,
Vladimir