[PATCH v2 00/10] drm: Support simple-framebuffer devices and firmware fbs

Thu Mar 25 11:29:57 UTC 2021

Hi,

On 3/18/21 11:29 AM, Thomas Zimmermann wrote:
> This patchset adds support for simple-framebuffer platform devices and
> a handover mechanism for native drivers to take-over control of the
> hardware.
> 
> The new driver, called simpledrm, binds to a simple-frambuffer platform
> device. The kernel's boot code creates such devices for firmware-provided
> framebuffers, such as EFI-GOP or VESA. Typically the BIOS, UEFI or boot
> loader sets up the framebuffers. Description via device tree is also an
> option.
> 
> Simpledrm is small enough to be linked into the kernel. The driver's main
> purpose is to provide graphical output during the early phases of the boot
> process, before the native DRM drivers are available. Native drivers are
> typically loaded from an initrd ram disk. Occationally simpledrm can also
> serve as interim solution on graphics hardware without native DRM driver.
> 
> So far distributions rely on fbdev drivers, such as efifb, vesafb or
> simplefb, for early-boot graphical output. However fbdev is deprecated and
> the drivers do not provide DRM interfaces for modern userspace.
> 
> Patches 1 and 2 prepare the DRM format helpers for simpledrm.
> 
> Patches 3 and 4 add a hand-over mechanism. Simpledrm acquires it's
> framebuffer's I/O-memory range and provides a callback function to be
> removed by a native driver. The native driver will remove simpledrm before
> taking over the hardware. The removal is integrated into existing helpers,
> so drivers use it automatically.
> 
> Patches 5 to 10 add the simpledrm driver. It's build on simple DRM helpers
> and SHMEM. It supports 16-bit, 24-bit and 32-bit RGB framebuffers. During
> pageflips, SHMEM buffers are copied into the framebuffer memory, similar
> to cirrus or mgag200. The code in patches 8 and 9 handles clocks and
> regulators. It's based on the simplefb drivers, but has been modified for
> DRM.

Thank you for your work on this, this is very interesting.

> I've also been working on fastboot support (i.e., flicker-free booting).
> This requires state-readout from simpledrm via generic interfaces, as
> outlined in [1]. I do have some prototype code, but it will take a while
> to get this ready. Simpledrm will then support it.
> 
> I've tested simpledrm with x86 EFI and VESA framebuffers, which both work
> reliably. The fbdev console and Weston work automatically. Xorg requires
> manual configuration of the device. Xorgs current modesetting driver does
> not work with both, platform and PCI device, for the same physical
> hardware. Once configured, X11 works. I looked into X11, but couldn't see
> an easy way of fixing the problem. With the push towards Wayland+Xwayland
> I expect the problem to become a non-issue soon. Additional testing has
> been reported at [2].
> 
> One cosmetical issue is that simpledrm's device file is card0 and the
> native driver's device file is card1. After simpledrm has been kicked out,
> only card1 is left. This does not seem to be a practical problem however.
> 
> TODO/IDEAS:
> 
> 	* provide deferred takeover

I'm not sure what you mean with this ?  Currently deferred-takeover is
handled in the fbcon code. Current flickerfree boot works like this
(assuming a single LCD panel in a laptop):

1. EFI/GOP sets up the framebuffer, draws a vendor logo
2. The bootloader runs in silent mode and does not touch anything gfx related
3. kernel boots, with a loglevel of 3 so only CRIT/EMERG messages are shown
2. efifb loads; and tells fbcon that a framebuffer is now available for it to "bind"
   to. Since CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER=y fbcon defers taking over
   the console and leaves the dummy-console driver in place (unless there have already
   been kernel messages logged, which there shouldn't because loglevel=3)
3. i915 loads, reads out the hw state compares this to the preferred-mode for the
   panel which it would set, they match, nothing happens. i915 takes ownership
   of the scanout-buffer set up by the GOP, but leaves it in place.
   i915 also removes the efifb /dev/fb0 and installs its own /dev/fb0 fbdev compat
   device, fbcon is notified of this, but is still deferred and leaves the dummy
   console driver in place as console driver.
4. Plymouth loads, allocates a new scan-out buffer at the panel's preferred resolution,
   plymouth reads the vendor-logo through the BGRT ACPI interface and fills the
   scanout-buffer with the vendor-logo + a spinner. Then plymouth installs the new
   scanout-buffer on the crtc, this is done atomically during vsync, so the user
   sees no changes, other then the spinner appearing
   (note the active VT is now in graphical mode)
5. From here on not flickering is a userspace problem

AFAICT this should work fine with simplekms too, unless it clears the screen
to black when it binds.

An addition to the above sequence, if at any time either the kernel or userspace
prints a message to the console; and at that time a fbdev is registered then fbcon
will takeover as the console driver from the dummy driver and it will start drawing
to the registered fbdev (1), destroying the framebuffer contents. Also if any messages
where printend while no fbdev was registered, then fbcon will takeover the console
as soon as a fbdev gets registered.

So since we already have deferred-takeover in the fbcon code, I wonder what you
mean when you are talking about "provide deferred takeover" for simplekms?

Regards,

Hans

1) Except when the VT has been switched to GFX mode when this happens, then fbcon
will delay using the fbdev until the VT is switched back to text mode.

p.s.

This has the interesting side effect then when logging into a desktop GUI session:
kernel -> plymouth -> gdm -> GNOME user session

There never is any output to the text-console and fbcon never takes-over, so on
many Laptops running say Fedora workstation the fbcon code is actually unused
until the user manually switches to another virtual-console to log in in
text-mode:

[hans at x1 ~]$ dmesg | grep -E 'fbcon|Console:|Truecolor'
[    0.258904] Console: colour dummy device 80x25
[    1.274726] efifb: Truecolor: size=8:8:8:8, shift=24:16:8:0
[    1.274768] fbcon: Deferring console take-over
[    2.540894] fbcon: i915drmfb (fb0) is primary device
[    2.540896] fbcon: Deferring console take-over
[hans at x1 ~]$ uptime
 12:29:39 up  4:19,  1 user,  load average: 0.58, 0.75, 0.81

Look mom no fbcon

> 	* provide bootsplash DRM client
> 	* make simplekms usable with ARM-EFI fbs
> 
> v2:
> 	* rename to simpledrm, aperture helpers
> 	* reorganized patches
> 	* use hotplug helpers for removal (Daniel)
> 	* added DT match tables (Rob)
> 	* use shadow-plane helpers
> 	* lots of minor cleanups
> 
> [1] https://lore.kernel.org/dri-devel/CAKMK7uHtqHy_oz4W7F+hmp9iqp7W5Ra8CxPvJ=9BwmvfU-O0gg@mail.gmail.com/
> [2] https://lore.kernel.org/dri-devel/1761762.3HQLrFs1K7@nerdopolis/
> 
> Thomas Zimmermann (10):
>   drm/format-helper: Pass destination pitch to drm_fb_memcpy_dstclip()
>   drm/format-helper: Add blitter functions
>   drm/aperture: Move fbdev conflict helpers into drm_aperture.h
>   drm/aperture: Add infrastructure for aperture ownership
>   drm: Add simpledrm driver
>   drm/simpledrm: Add fbdev emulation
>   drm/simpledrm: Initialize framebuffer data from device-tree node
>   drm/simpledrm: Acquire clocks from DT device node
>   drm/simpledrm: Acquire regulators from DT device node
>   drm/simpledrm: Acquire memory aperture for framebuffer
> 
>  Documentation/gpu/drm-internals.rst    |  12 +
>  MAINTAINERS                            |   7 +
>  drivers/gpu/drm/Kconfig                |   7 +
>  drivers/gpu/drm/Makefile               |   1 +
>  drivers/gpu/drm/drm_aperture.c         | 287 ++++++++
>  drivers/gpu/drm/drm_format_helper.c    |  96 ++-
>  drivers/gpu/drm/mgag200/mgag200_mode.c |   2 +-
>  drivers/gpu/drm/tiny/Kconfig           |  17 +
>  drivers/gpu/drm/tiny/Makefile          |   1 +
>  drivers/gpu/drm/tiny/cirrus.c          |   2 +-
>  drivers/gpu/drm/tiny/simpledrm.c       | 932 +++++++++++++++++++++++++
>  include/drm/drm_aperture.h             |  96 +++
>  include/drm/drm_fb_helper.h            |  56 +-
>  include/drm/drm_format_helper.h        |  10 +-
>  14 files changed, 1466 insertions(+), 60 deletions(-)
>  create mode 100644 drivers/gpu/drm/drm_aperture.c
>  create mode 100644 drivers/gpu/drm/tiny/simpledrm.c
>  create mode 100644 include/drm/drm_aperture.h
> 
> --
> 2.30.1
> 