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

From: Archit Taneja
Date: Tue Mar 10 2015 - 06:22:55 EST

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:
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@xxxxxxxxxxxxxx>
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
FBDEV helpers for KMS drivers.

+ bool "Enable legacy fbdev support for your modesetting driver"
+ depends on DRM
+ select FB_SYS_FOPS
+ 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

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.

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.


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