F36 Change proposal: Replace the fbdev drivers with simpledrm and the DRM fbdev emulation layer (System-Wide Change proposal)

[Ben Cotton]

https://fedoraproject.org/wiki/Changes/ReplaceFbdevDrivers

== Summary ==

This change replaces the legacy Linux frame buffer device (fbdev)
drivers that are still used in Fedora, with the latest simpledrm
driver and the DRM fbdev emulation layer.

== Owner ==

* Name: [[User:Javierm|Javier Martinez Canillas]]
* Email: javierm(a)redhat.com

* Name: [[User:Pbrobinson|Peter Robinson]]
* Email: pbrobinson(a)fedoraproject.org

== Detailed Description ==

The fbdev subsystem has been deprecated for over a decade and no new
platform should use it but instead write DRM drivers for their video
output.

Yet, the Fedora kernel still enables support for some of the fbdev
drivers. Mainly for fbdev drivers that make use of the video frame
buffer that was initialized by the firmware. These are generic drivers
that could be used in different platforms since only grab an existing
frame buffer without trying to do any hardware initialization and
setup.

Examples of these drivers are simplefb (used in aarch64 machines with
Device Trees) and efifb (used in EFI machines) and vesafb (used in
machines with a VESA VGA graphics). But since the Linux v5.14, a new
simpledrm driver was introduced that is able to do the same that these
fbdev drivers. And just like any other DRM driver, it can rely on the
DRM subsystem fbdev emulation layer to provide a fbdev interface.

For this reason, there is no need for the fbdev drivers anymore and
instead the simpledrm driver could just be used.

== Benefit to Fedora ==

This change will allow to reduce the maintenance burden in the Linux
kernel since users won't use legacy fbdev drivers anymore but instead
a driver in the well maintained DRM subsystem. This will also move
Fedora one step close to being able to completely disable the fbdev
subsystem.

The latter is still not possible due the framebuffer console (fbcon)
needing to run on top of a frame buffer device. But at least this
change will remove the dependency with fbdev drivers and make fbcon
just use the DRM fbdev emulation layer in the meantime.

== Scope ==

* Proposal owners:
** Modify the following Linux kernel config options:
*** Disable fbdev drivers (e.g: `CONFIG_FB_EFI`, `CONFIG_FB_SIMPLE`) options.
*** Make `CONFIG_DRM` option built-in so the display can be enabled
without an initrd.
*** Enable the `CONFIG_SYSFB_SIMPLEFB` that registers a
simple-framebuffer platform device that is used to match the simpledrm
driver.
* Other developers:
** It should not affect other packages but may be that plymouth or
display managers could need some work.
**  Test and watch for regressions.

* Release engineering: []
* Policies and guidelines:  No changes needed.
* Trademark approval: No changes needed.

== Upgrade/compatibility impact ==

There should not be any impact on upgrade, this is a internal change
in the kernel and should be transparent for users.

== How To Test ==

* Install Fedora 36 and check that there is output in the video
console when removing the `rhgb quiet` parameters.

== User Experience ==
No visible changes are expected, as mentioned it is fully backward
compatible and transparent for users.

== Dependencies ==
None

== Contingency Plan ==

* Contingency mechanism: Revert the Linux kernel configuration changes.
* Contingency deadline: Beta Freeze
* Blocks release? No
* Blocks product? None

== Documentation ==
* N/A


-- 
Ben Cotton
He / Him / His
Fedora Program Manager
Red Hat
TZ=America/Indiana/Indianapolis

[Neal Gompa]

On Tue, Oct 26, 2021 at 11:27 AM Ben Cotton <bcotton(a)redhat.com&gt; wrote:
>  https://fedoraproject.org/wiki/Changes/ReplaceFbdevDrivers
>
>  == Summary ==
>
>  This change replaces the legacy Linux frame buffer device (fbdev)
>  drivers that are still used in Fedora, with the latest simpledrm
>  driver and the DRM fbdev emulation layer.
>  
I'd like to see this coupled with the addition of a flag so we can
change "basic graphics mode" to force this. There's already a proposed
patch series upstream for this[1], so I think this isn't too hard to
achieve.

This would unblock closing the remaining gaps on the X11->Wayland
transition, since Mutter doesn't support fbdev for Wayland, and KWin's
Wayland session doesn't work with fbdev on our kernel for some reason.

[1]: https://lists.freedesktop.org/archives/dri-devel/2021-October/328704.html



-- 
真実はいつも一つ！/ Always, there's only one truth!

[Javier Martinez Canillas]

On Tue, Oct 26, 2021 at 6:01 PM Neal Gompa <ngompa13(a)gmail.com&gt; wrote:
>  On Tue, Oct 26, 2021 at 11:27 AM Ben Cotton <bcotton(a)redhat.com&gt; wrote:
>  >
>  > https://fedoraproject.org/wiki/Changes/ReplaceFbdevDrivers
>  >
>  > == Summary ==
>  >
>  > This change replaces the legacy Linux frame buffer device (fbdev)
>  > drivers that are still used in Fedora, with the latest simpledrm
>  > driver and the DRM fbdev emulation layer.
>  >
>
>  I'd like to see this coupled with the addition of a flag so we can
>  change "basic graphics mode" to force this. There's already a proposed
>  patch series upstream for this[1], so I think this isn't too hard to
>  achieve.
>  
Agreed. I've set up a COPR that has a kernel build with the needed
config changes and the patches you mentioned. In case someone wants to
give a try to this:

https://copr.fedorainfracloud.org/coprs/javierm/simpledrm/

>  This would unblock closing the remaining gaps on the X11->Wayland
>  transition, since Mutter doesn't support fbdev for Wayland, and KWin's
>  Wayland session doesn't work with fbdev on our kernel for some reason.
>  
Indeed, I think that using simpledrm would be an improvement over efifb.

Best regards,
Javier

[Adam Williamson]

On Tue, 2021-10-26 at 11:26 -0400, Ben Cotton wrote:
>  https://fedoraproject.org/wiki/Changes/ReplaceFbdevDrivers
>  
>  * Other developers:
>  ** It should not affect other packages but may be that plymouth or
>  display managers could need some work.
>  **  Test and watch for regressions.
>  
>  * Release engineering: []
>  * Policies and guidelines:  No changes needed.
>  * Trademark approval: No changes needed.
>  
>  == Upgrade/compatibility impact ==
>  
>  There should not be any impact on upgrade, this is a internal change
>  in the kernel and should be transparent for users.
>  
>  == How To Test ==
>  
>  * Install Fedora 36 and check that there is output in the video
>  console when removing the `rhgb quiet` parameters.
>  
>  == User Experience ==
>  No visible changes are expected, as mentioned it is fully backward
>  compatible and transparent for users. 
This all seems to downplay things a bit. Following on from Neal's point
about basic graphics mode, whether we change that menu item to
explicitly use this new mechanism or not, it seems entirely plausible
that the behavior of login managers in any case but 'working native
driver' will need testing - what happens when we explicitly boot to
this new mechanism? What happens when we boot with 'nomodeset', which
people may continue to use even if we change what the menu options do?
What happens on KDE, GNOME, and (less importantly) non-release-blocking
desktops?

This is an area that's known to cause problems, so I wouldn't want to
assume everything will just be fine.

Overall it sounds like an improvement, though, so I hope it works out.
-- 
Adam Williamson
Fedora QA
IRC: adamw | Twitter: adamw_ha
https://www.happyassassin.net