From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>
Subject: Re: [PATCH v4 10/10] pwm-backlight: Add rudimentary device tree support
Date: Tue, 20 Mar 2012 09:27:04 -0600
Message-ID: <4F68A1C8.5080608@wwwdotorg.org>
References: <1331740593-10807-1-git-send-email-thierry.reding@avionic-design.de> <1331740593-10807-11-git-send-email-thierry.reding@avionic-design.de> <4F67F2AF.3020404@wwwdotorg.org> <20120320083943.GA20249@avionic-0098.adnet.avionic-design.de>
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To: Thierry Reding <thierry.reding-RM9K5IK7kjKj5M59NBduVrNAH6kLmebB@public.gmane.org>
Cc: devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-tegra-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Sascha Hauer <s.hauer-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>, Matthias Kaehlcke <matthias-RprLehDfhQ3k1uMJSBkQmQ@public.gmane.org>, Kurt Van Dijck <kurt.van.dijck-/BeEPy95v10@public.gmane.org>, Rob Herring <rob.herring-bsGFqQB8/DxBDgjK7y7TUQ@public.gmane.org>, Grant Likely <grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org>, Colin Cross <ccross-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>, Olof Johansson <olof-nZhT3qVonbNeoWH0uzbU5w@public.gmane.org>, Richard Purdie <rpurdie-Fm38FmjxZ/leoWH0uzbU5w@public.gmane.org>, Mark Brown <broonie-yzvPICuk2AATkU/dhu1WVueM+bqZidxxQQ4Iyu8u01E@public.gmane.org>, Mitch Bradley <wmb-D5eQfiDGL7eakBO8gow8eQ@public.gmane.org>, Mike Frysinger <vapier-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org>, Eric Miao <eric.y.miao-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Lars-Peter Clausen <lars-Qo5EllUWu/uELgA04lAiVw@public.gmane.org>, Ryan Mallon <rmallon-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
List-Id: linux-tegra@vger.kernel.org

On 03/20/2012 02:39 AM, Thierry Reding wrote:
> * Stephen Warren wrote:
>> On 03/14/2012 09:56 AM, Thierry Reding wrote:
>>> This commit adds very basic support for device tree probing. Currently,
>>> only a PWM and a list of distinct brightness levels can be specified.
>>> Enabling or disabling backlight power via GPIOs is not yet supported.
>>>
>>> A pointer to the exit() callback is stored in the driver data to keep it
>>> around until the driver is unloaded.
...
>>>  static int pwm_backlight_probe(struct platform_device *pdev)
>> ...
>>> -	pb->pwm = pwm_request(data->pwm_id, "backlight");
>>> -	if (IS_ERR(pb->pwm)) {
>>> -		dev_err(&pdev->dev, "unable to request PWM for backlight\n");
>>> -		ret = PTR_ERR(pb->pwm);
>>> -		goto err_alloc;
>>> -	} else
>>> -		dev_dbg(&pdev->dev, "got pwm for backlight\n");
>>> +	if (!pb->pwm) {
>>> +		pb->pwm = pwm_request(data->pwm_id, "backlight");
>>> +		if (IS_ERR(pb->pwm)) {
>>> +			dev_err(&pdev->dev, "unable to request PWM for backlight\n");
>>> +			ret = PTR_ERR(pb->pwm);
>>> +			goto err_alloc;
>>> +		} else
>>> +			dev_dbg(&pdev->dev, "got pwm for backlight\n");
>>> +	}
>>
>> Hmmm. It'd be more consistent if pwm_backlight_parse_dt() called
>> something like of_pwm_get() instead of of_pwm_request(), so that this
>> code could always call pwm_request() on the PWM and hence operate the
>> same irrespective of DT vs non-DT. GPIOs work that way at least.
> 
> That's actually what the initial patch had. Unfortunately that's pretty much
> the opposite direction of where the PWM framework is headed because it would
> involve getting a global index to request the PWM.

Not necessarily; get() could return a controller+index pair, which could
then be passed to request().

> I think in the long run it
> would be much better to get rid of pwm_request() altogether and unify by
> having the non-DT case request the PWM device on a per-chip basis.

That might also work.