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

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Message-ID: <20150311151731.GJ3800@phenom.ffwll.local>
Date:	Wed, 11 Mar 2015 16:17:31 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Archit Taneja <architt@...eaurora.org>
Cc:	robdclark@...il.com, airlied@...ux.ie, treding@...dia.com,
	p.zabel@...gutronix.de, benjamin.gaignard@...aro.org,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	Jani Nikula <jani.nikula@...ux.intel.com>
Subject: Re: [RFC 1/6] drm: Add top level Kconfig option for DRM fbdev
 emulation

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?

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