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

From: Archit Taneja
Date: Fri Mar 13 2015 - 02:26:29 EST

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
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.

But with the stubbed out functions that should work, right? Why doesn't

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:







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?".

I'll give this a try. Although, it would be a better idea to make the drivers not call these at all when fbdev emulation isn't asked for. With the stubs, if someone does try to use the driver with DRM_FBDEV_EMULATION not set, the worst that'll happen would be that the driver fails to probe. Which isn't so bad.


Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at