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

From: Archit Taneja
Date: Mon Mar 09 2015 - 04:54:20 EST

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@xxxxxxxxxxxxxx> 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.

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.


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