linux-kernel - Re: [PATCH] drm: msm: Fix build when legacy fbdev support isn't set

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Date:	Mon, 09 Mar 2015 14:24:09 +0530
From:	Archit Taneja <architt@...eaurora.org>
To:	Rob Clark <robdclark@...il.com>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	daniel@...ll.ch
Subject: Re: [PATCH] drm: msm: Fix build when legacy fbdev support isn't set



On 03/09/2015 01:14 PM, Daniel Vetter wrote:
> On Mon, Mar 09, 2015 at 11:27:06AM +0530, Archit Taneja wrote:
>> On 03/05/2015 09:14 PM, Daniel Vetter wrote:
>>> On Thu, Mar 05, 2015 at 07:10:44AM -0500, Rob Clark wrote:
>>>> On Thu, Mar 5, 2015 at 5:06 AM, Archit Taneja <architt@...eaurora.org> wrote:
>>>>>
>>>>> On 02/23/2015 09:09 PM, Daniel Vetter wrote:
>>>>>>
>>>>>> On Mon, Feb 23, 2015 at 10:03:21AM -0500, Rob Clark wrote:
>>>>> Rather than creating fb helper stub functions, maybe we could help each drm
>>>>> driver create two variants of functions needed by drm core(like
>>>>> output_poll_changed and dev_lastclose), one variant supporting legacy fbdev,
>>>>> and the other not?
>>>>
>>>> So one quick thought.. building without fbdev would ideally/eventually
>>>> be a distro level decision, not a driver level decision..  so I think
>>>> it is *eventually* not a problem for it being a global drm level
>>>> decision.  The only problem is right now some drivers support no-fbdev
>>>> config, and some do not.  Maybe it is worth fixing that?
>>>
>>> Yeah, if we get fbdev emulation Kconfig option then I think i915 and msm
>>> should remove their own options and just use that. There's really no need
>>> to have this per-driver, this is a question of what userspace expects and
>>> so per-distro, independant of the driver.
>>> -Daniel
>>>
>>
>> Okay. If I understand right. We need to do something like this:
>>
>> - Create a new Kconfig option that lets us emulate fbdev
>>
>> - For drivers that already have a config for fbdev emulation, replace it
>> with our new emulation config.
>>
>> - For drivers that assume fbdev emulation by default, select our new
>> emulation config in their respective Kconfigs.
>>
>> Does this sound okay?
>>
>> I suppose this could be the first step. Later we'd need to work on each
>> driver to work with and without the fbdev emulation Kconfig option.
>
> See Rob's idea quoted above: Imo you should just do the conditional code
> in the fbdev emulation helper in drm_fb_helper.c, not in drivers. There's
> some additional code in i915 (and maybe also msm) to compile out (mostly
> around taking/dropping console_lock) and we want to keep that. But that
> should also just be using the same new config option. So:
>
> - Add new config option DRM_FBDEV_EMULATION (default y for backwards
>    compat). Use that to stub out helpers in drm_fb_helper.c.
>
> - Replace the msm/i915 specific options with the new, i.e. remove it from
>    Kconfig and use DRM_FBDEV_EMULATION in the code instead of the msm/i915
>    specific one.
>
> fbdev-less support for all other drivers will just magically happen
> without the need for driver-specific code (except for the special locking
> in i915 and similar things maybe).

That sounds good. I wasn't totally sure about fbdev-less support working 
out of the box on devices that use the fb CMA helpers. The 
drm_fbdev_cma_* funcs internally use the fb helper functions. Stubbing 
the fb helpers out might result in some corner cases.

However, after a quick look at that the cma helpers, it looks like it 
take all the necessary precautions to prevent uninitialized accesses. 
Although, it would be ideal to stub out the drm_fbdev_cma_* helper 
functions too since they don't serve any purpose when fbdev emulation is 
disabled.

Archit


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