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

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Date:	Tue, 10 Mar 2015 11:05:47 +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, daniel@...ll.ch,
	daniel.vetter@...ll.ch
Subject: Re: [RFC 1/6] drm: Add top level Kconfig option for DRM fbdev
 emulation

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:
> >>Legacy fbdev emulation support via DRM is achieved through KMS FB helpers.
> >>Most modesetting drivers enable provide fbdev emulation by default by selecting
> >>KMS FB helpers. A few provide a separate Kconfig option for the user to enable
> >>or disbale fbdev emulation.
> >>
> >>Enabling fbdev emulation is finally a distro-level decision. Having a top level
> >>Kconfig option for fbdev emulation helps by providing a uniform way to
> >>enable/disable fbdev emulation for any modesetting driver. It also lets us
> >>remove unnecessary driver specific Kconfig options that causes bloat.
> >>
> >>With a top level Kconfig in place, we can stub out the fb helper functions when
> >>not needed without breaking functionality. Having stub functions also prevents
> >>drivers to require wrapping fb helper function calls with #ifdefs.
> >>
> >>DRM_FBDEV_EMULATION defaults to y since many drivers enable fbdev emulation by
> >>default and majority of distributions expect the fbdev interface in the kernel.
> >>
> >>For now, this config selects both FB_SYS_* and FB_CFB_* configs as some
> >>modesetting drivers use the former and other the later. This needs to be taken
> >>care of in a better way.
> >>
> >>Signed-off-by: Archit Taneja <architt@...eaurora.org>
> >>---
> >>  drivers/gpu/drm/Kconfig     |  18 +++++++
> >>  drivers/gpu/drm/Makefile    |   4 ++
> >>  include/drm/drm_fb_helper.h | 120 ++++++++++++++++++++++++++++++++++++++++++++
> >>  3 files changed, 142 insertions(+)
> >>
> >>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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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