linux-kernel - Re: [PATCH] Revert "backlight: pwm: Handle EPROBE

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Date:	Thu, 01 Oct 2015 11:06:33 +0200
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Boris BREZILLON <boris.brezillon@...e-electrons.com>,
	Thierry Reding <thierry.reding@...il.com>
Cc:	<linux-pwm@...r.kernel.org>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Lee Jones <lee.jones@...aro.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	<linux-fbdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Revert "backlight: pwm: Handle EPROBE_DEFER while requesting the PWM"

Nicolas Ferre <nicolas.ferre@...el.com> writes:

> Le 30/09/2015 21:29, Robert Jarzmik a écrit :
>> Robert Jarzmik <robert.jarzmik@...e.fr> writes:
>> 
>>> This reverts commit 68feaca0b13e453aa14ee064c1736202b48b342f.
>>> This commit breaks legacy platforms, for which :
>>>  (a) no pwm table is added (legacy platforms)
>>>  (b) in this case, in pwm_get(), pmw_lookup_list is empty, and therefore
>>>      chosen == NULL, and therefore pwm_get() returns NULL, and pwm_get()
>>>      returns -EPROBE_DEFER
>>>  (c) as a consequence, this code is unreachable in pwm_bl.c :
>>>      if (IS_ERR(pb->pwm)) {
>>> 	ret = PTR_ERR(pb->pwm);
>>>  	dev_info(&pdev->dev, "%s:%d(): %d\n", __func__, __LINE__, ret);
>>>  	if (ret == -EPROBE_DEFER)
>>>  		goto err_alloc;
>>>
>>>  	dev_err(&pdev->dev, "unable to request PWM, trying legacy API\n");
>>>  	pb->legacy = true;
>>>  	pb->pwm = pwm_request(data->pwm_id, "pwm-backlight");
>>>
>>> As this code is unreachable, all legacy platforms relying on pwm_id are
>>> broken, amongst which pxa have been tested as broken.
>>>
>>> Signed-off-by: Robert Jarzmik <robert.jarzmik@...e.fr>
>> Thierry, would you have a look please ?
>> As I said before, all legacy platform relying on pwm_id are broken. I'd like to
>> be sure this lands in the next -rc series.
>
> Well, as I answered on the linux-pwm mailing-list (I was not in copy) here:
> http://article.gmane.org/gmane.linux.pwm/2744
> I wonder if it's not easier to fix the platforms and add the pwm tables...
No it's not, at least not for a -rc cycle. It's the long term solution you're
talking about, not the fix one.

> Otherwise, Boris proposed this fix:
> 8<-----------------------------------------------------------
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index eff379b..00483d4 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -273,15 +273,15 @@ 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;
>  
>  		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);
> +			if (ret != -EPROBE_DEFER)
> +				ret = PTR_ERR(pb->pwm);
> +
>  			goto err_alloc;
>  		}
>  	}
>
> which is not tested and may add an extra non-valid error log.
I can test that, today, it looks an interesting alternative.

If both solutions do work, someone (Boris) can post a patch for this -rc instead
of the revert. If no patch is posted, I maintain my Revert, as this patch _does_
break platforms (omap is broken too AFAICS).

Cheers.

-- 
Robert

PS: I have not received http://article.gmane.org/gmane.linux.pwm/2744.
    Is it my mailer or MUA which is broken, ie. was I in the "To:" of the mail ?

PPS: Sorry to having forgotten to join you to the revert

PPPS: As long as an other patch is not submitted to fix the issue (other than
      the Revert), I NAK the NAK. There is a breakage introduced, and I consider
      it a strong enough reason to be maintained for the -rc serie.

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