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

From: Archit Taneja <architt@codeaurora.org>
To: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Tomi Valkeinen <tomi.valkeinen@ti.com>,
	Thierry Reding <treding@nvidia.com>
Subject: Re: [RFC 1/6] drm: Add top level Kconfig option for DRM fbdev emulation
Date: Tue, 30 Jun 2015 14:36:44 +0530	[thread overview]
Message-ID: <55925C24.8030909@codeaurora.org> (raw)
In-Reply-To: <CA+M3ks4vtDfA3rxBhEoiuZ8Tv_k15ddHRvAWP+MNgLkOMArTaw@mail.gmail.com>

Hi,

On 06/30/2015 02:01 PM, Benjamin Gaignard wrote:
> 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 ?

I guess this won't be hard to do once we have a common fbdev emulation 
config option. We could consider this as a drm top level module param.

I'll keep this in mind.

Thanks,
Archit

>
> Benjamin
>
> 2015-06-30 9:56 GMT+02:00 Archit Taneja <architt@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@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
>
>
>

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project
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