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

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Date:	Fri, 13 Mar 2015 10:06:51 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Archit Taneja <architt@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	Jani Nikula <jani.nikula@...ux.intel.com>, daniel@...ll.ch,
	robdclark@...il.com, airlied@...ux.ie, treding@...dia.com,
	p.zabel@...gutronix.de, benjamin.gaignard@...aro.org,
	dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
	tomi.valkeinen@...com
Subject: Re: [RFC 1/6] drm: Add top level Kconfig option for DRM fbdev
 emulation

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.

What do you think?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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