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SubjectRe: [RFC 1/6] drm: Add top level Kconfig option for DRM fbdev emulation

On 03/11/2015 08:47 PM, Daniel Vetter wrote:
> On Wed, Mar 11, 2015 at 01:51:02PM +0530, Archit Taneja wrote:
>> On 03/10/2015 05:47 PM, Daniel Vetter wrote:
>>> On Tue, Mar 10, 2015 at 03:52:41PM +0530, Archit Taneja wrote:
>>>> 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:
>>>>>>>> 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.
>>> But with the stubbed out functions that should work, right? Why doesn't
>>> it?
>> There are still calls to functions from fb core like fb_set_suspend and
>> register_framebuffer which aren't covered by the drm fb helper functions.
> Hm, sounds like we need another patch to stub out fb_set_suspend when
> fbdev isn't enabled. Is there anything else?

There are a handful of fb core functions which are called by drm drivers:







>>>> 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.
>>> The problem with the patch as-is the massive amounts of selects the FB
>>> helper still has. We need to get rid of them so that when you disable
>>> fbdev emulation you can indeed disable fbcon and the entire fbdev
>>> subsystem. I've thought that remove the hidden symbol DRM_KMS_FB_HELPER
>>> and moving all the selects to DRM_FBDEV_EMULATION should work out? If that
>>> doesn't work we need to look again how to better stub things out I think.
>> That's true. Also, as Jani pointed out, maybe it isn't the best idea to make
>> every kms driver select DRM_FBDEV_EMULATION?
>> Maybe the drivers that are currently built with fbdev emulation by default
>> can add a 'depends on DRM_FBDEV_EMULATION'? Since the config defaults to y,
>> the drivers should run as is. Later, we could take up each driver and build
>> the fb stuff only when DRM_FBDEV_EMULATION is set, like how we do for i915
>> and msm?
> Yeah we definitely can't mix select with a user-visible option. I think we
> just need to fix up the remaining functions and create stubs for them if
> needed and then drop all the selects. Well in DRM_FBDEV_EMULATION we
> should still select for fbcon since otherwise tons of bug reports about
> "where is my fbcon with kms?".

I'll give this a try. Although, it would be a better idea to make the
drivers not call these at all when fbdev emulation isn't asked for. With
the stubs, if someone does try to use the driver with
DRM_FBDEV_EMULATION not set, the worst that'll happen would be that the
driver fails to probe. Which isn't so bad.


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