[PATCH v2 00/10] drm: Support simple-framebuffer devices and firmware fbs
Thomas Zimmermann
tzimmermann at suse.de
Mon Mar 29 12:31:12 UTC 2021
Hi
Am 25.03.21 um 12:29 schrieb Hans de Goede:
> 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.
I forgot to add the code that clears the screen, but that's the case here.
Instead of a plane-disable operation, simpledrm can at best clear the
screen. This would happen during the initial mode-config reset IIRC. But
we need to keep the display content stored in a framebuffer, so read-out
helpers are required. There are more users of these read-out helpers.
Adding them at some point probably makes sense.
Other drivers might also want to read the initial config from simpledrm
via read-out helpers. I think only i915 currently supports something
like that ATM.
Best regards
Thomas
>
> 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
>>
>
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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