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

[Archit Taneja]

Hi,

On 06/30/2015 12:40 PM, Daniel Vetter wrote:
> Any updates on this or too much distractions? I really think doing
> this would be awesome for the drm subsystem, instead of reinventing
> this wheel for each driver.

I'd started on this again. I've got more free time now than before, so I 
should have something in a week or so. I agree it will help a lot (there 
are already two new drivers since we started discussing this!)

Archit

> -Daniel
>
> On Wed, Mar 25, 2015 at 10:21 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
>> On Wed, Mar 25, 2015 at 01:47:54PM +0530, Archit Taneja wrote:
>>> Hi,
>>>
>>> On 03/13/2015 02:36 PM, Daniel Vetter wrote:
>>>> On Fri, Mar 13, 2015 at 11:55:07AM +0530, Archit Taneja wrote:
>>>>>
>>>>>
>>>>> 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:
>>>>>
>>>>> fb_alloc_cmap/fb_dealloc_cmap
>>>>>
>>>>> fb_sys_read/fb_sys_write
>>>>>
>>>>> register_framebuffer/unregister_framebuffer/unlink_framebuffer/
>>>>> remove_conflicting_framebuffers
>>>>>
>>>>> fb_set_suspend
>>>>>
>>>>> fb_deferred_io_init/fb_deferred_io_cleanup
>>>>>
>>>>> framebuffer_alloc/framebuffer_release
>>>>
>>>> Hm yeah that's somewhat annoying indeed. What about the following:
>>>> 1. We move all the #include <linux/fb.h> from drivers into drm_fb_helper.h
>>>>
>>>> 2. Then we add stubs for these functions in drm_fb_helper.h, like this
>>>>
>>>> #if defined(CONFIG_FB)
>>>> #include <linux/fb.h>
>>>> #else
>>>>
>>>> /* static inline stubs for all the fb stuff used by kms drivers */
>>>> #endif
>>>>
>>>> Imo this makes sense since kms drivers really have a bit a special
>>>> situation with fbdev. They're not full-blown fbdev drivers and can be
>>>> useful fully without fbdev.
>>>>
>>>
>>> I was trying this out. Removing 'linux/fb.h' and replacing stub fb funcs
>>> won't really work because struct declarations(like fb_info) also get
>>> removed.
>>>
>>> I considered placing '#if IS_ENABLED(CONFIG_FB)' within linux/fb.h itself,
>>> but that seemed a bit too intrusive.
>>>
>>> This is what I'm currently doing:
>>>
>>> - Some funcs, like framebufer_alloc/release, alloc_cmap/dealloc_cmap would
>>> actually benefit if we have drm fb helpers for them. They are used in
>>> exactly the same manner by all the drivers.
>>>
>>> - For the rest of the functions that are sparsely used, I was considering
>>> making very simple drm_fb_* wrapper functions. Something like:
>>>
>>> void drm_fb_helper_deferred_io_init(struct drm_fb_helper *helper)
>>> {
>>>        if (helper->fbdev)
>>>                fb_deferred_io_init(helper->fbdev);
>>> }
>>>
>>> We could have all fb calls called within drm_fb_helper.c, creating
>>> drm_fb_helper_* stub functions would then be an easier task. What do you
>>> think?
>>
>> That's actually an option I considered, but I hoped we could do it with
>> less work. If this indeed works and you can create the patch that would be
>> awesome.
>>
>> Thanks, Daniel
>> --
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch
>
>
>

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