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

[Daniel Vetter <daniel@ffwll.ch>]

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?

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