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

Tue Jun 30 01:31:35 PDT 2015

Hi,

I think that what have been done by Rob with module_param is also a good idea:
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/drivers/gpu/drm/msm/msm_drv.c?id=e90dfec78ec288d6c89a7b508a5c5d4ae8b7f934

Can you mix compilation flag and module param ?

Benjamin

2015-06-30 9:56 GMT+02:00 Archit Taneja <architt at codeaurora.org>:
> 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
>>
>>
>>
>>
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> a Linux Foundation Collaborative Project

-- 
Benjamin Gaignard

Graphic Working Group

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