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

Hans de Goede hdegoede at redhat.com
Tue Mar 30 08:34:12 UTC 2021


Hi,

On 3/30/21 9:09 AM, Thomas Zimmermann wrote:
> Hi
> 
> Am 29.03.21 um 16:50 schrieb Hans de Goede:
>> Hi,
>>
>> On 3/29/21 2:31 PM, Thomas Zimmermann wrote:
>>> 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.
>>
>> Hmm, clearing the screen without any (atomic) modesets being initiated by either
>> an in kernel drm-client or userspace seems wrong, this is certainly different from
>> what the i915 driver does. The way I see it either a drm client provides a new
>> framebuffer in which case you copy that over the old contents, effectively clearing
>> it. Or a drm-client gets a handle and draws to the current fb directly, in which
>> case it is the clients responsibility to clear it itself first.
>>
>> IOW I believe that simpledrm should not clear the screen itself at all.
> 
> I do as well. And when I boot with simpledrm + native driver it's flicker-free from what I can tell. But drm_mode_config_reset() is supposed to reset HW and software state. There could be some corner case where we'd have to clear the screen. For now, it should be fine.

Sounds good, thanks.

> BTW if you have the time I'd appreciate your review of the patchset.

Sorry, but I'm burried under a whole pile of other work. So although I would like to help it is better to say no.

Regards,

Hans





>>> 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
>>>>>
>>>>
>>>
>>
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