linux-kernel - Re: [RFC 1/6] drm: Add top level Kconfig option for DRM fbdev emulation

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Message-ID: <54FEC5F1.90208@codeaurora.org>
Date:	Tue, 10 Mar 2015 15:52:41 +0530
From:	Archit Taneja <architt@...eaurora.org>
To:	robdclark@...il.com, airlied@...ux.ie, treding@...dia.com,
	p.zabel@...gutronix.de, benjamin.gaignard@...aro.org,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, daniel@...ll.ch
Subject: Re: [RFC 1/6] drm: Add top level Kconfig option for DRM fbdev emulation



On 03/10/2015 03:35 PM, Daniel Vetter wrote:
> On Tue, Mar 10, 2015 at 03:22:49PM +0530, Archit Taneja wrote:
>>
>>
>> On 03/10/2015 03:17 PM, Daniel Vetter wrote:
>>> On Tue, Mar 10, 2015 at 03:11:28PM +0530, Archit Taneja wrote:
>>>> Legacy fbdev emulation support via DRM is achieved through KMS FB helpers.
>>>> Most modesetting drivers enable provide fbdev emulation by default by selecting
>>>> KMS FB helpers. A few provide a separate Kconfig option for the user to enable
>>>> or disbale fbdev emulation.
>>>>
>>>> Enabling fbdev emulation is finally a distro-level decision. Having a top level
>>>> Kconfig option for fbdev emulation helps by providing a uniform way to
>>>> enable/disable fbdev emulation for any modesetting driver. It also lets us
>>>> remove unnecessary driver specific Kconfig options that causes bloat.
>>>>
>>>> With a top level Kconfig in place, we can stub out the fb helper functions when
>>>> not needed without breaking functionality. Having stub functions also prevents
>>>> drivers to require wrapping fb helper function calls with #ifdefs.
>>>>
>>>> DRM_FBDEV_EMULATION defaults to y since many drivers enable fbdev emulation by
>>>> default and majority of distributions expect the fbdev interface in the kernel.
>>>>
>>>> For now, this config selects both FB_SYS_* and FB_CFB_* configs as some
>>>> modesetting drivers use the former and other the later. This needs to be taken
>>>> care of in a better way.
>>>>
>>>> Signed-off-by: Archit Taneja <architt@...eaurora.org>
>>>> ---
>>>>   drivers/gpu/drm/Kconfig     |  18 +++++++
>>>>   drivers/gpu/drm/Makefile    |   4 ++
>>>>   include/drm/drm_fb_helper.h | 120 ++++++++++++++++++++++++++++++++++++++++++++
>>>>   3 files changed, 142 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
>>>> index 151a050..38f83a0 100644
>>>> --- a/drivers/gpu/drm/Kconfig
>>>> +++ b/drivers/gpu/drm/Kconfig
>>>> @@ -40,6 +40,24 @@ config DRM_KMS_FB_HELPER
>>>>   	help
>>>>   	  FBDEV helpers for KMS drivers.
>>>>
>>>> +config DRM_FBDEV_EMULATION
>>>> +	bool "Enable legacy fbdev support for your modesetting driver"
>>>> +	depends on DRM
>>>> +	select DRM_KMS_HELPER
>>>> +	select DRM_KMS_FB_HELPER
>>>> +	select FB_SYS_FILLRECT
>>>> +	select FB_SYS_COPYAREA
>>>> +	select FB_SYS_IMAGEBLIT
>>>> +	select FB_SYS_FOPS
>>>> +	select FB_CFB_FILLRECT
>>>> +	select FB_CFB_COPYAREA
>>>> +	select FB_CFB_IMAGEBLIT
>>>> +	default y
>>>> +	help
>>>> +	  Choose this option if you have a need for the legacy fbdev
>>>> +	  support. Note that this support also provide the linux console
>>>> +	  support on top of your modesetting driver.
>>>
>>> Maybe clarify that for linux console support you also need
>>> CONFIG_FRAMEBUFFER_CONSOLE? fbdev alone isn't enough.
>>
>> DRM_KMS_FB_HELPER selects that for us, right?
>
> Hm right I've missed that. Reminds me that you need one more patch at the
> end to remove all the various select DRM_KMS_FB_HELPER from all drm
> drivers. Otherwise this knob here won't work by default if you e.g. select
> radeon. In general we can't mix explicit options with menu entries with a
> select.

I was trying that out. Removing DRM_KMS_FB_HELPER and having 
DRM_FBDEV_EMULATION disabled breaks drivers which use FB stuff 
internally in their respective xyz_fbdev.c files.

Are you saying that we should remove DRM_KMS_FB_HELPER for such drivers 
and replace them with 'select DRM_FBDEV_EMULATION'?

Another option would be to provide #ifdef DRM_FBDEV_EMULATION 
wrap-arounds for fb related function calls/declarations in each driver, 
something that's already done for i915 and msm drivers.

Archit

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