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

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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.

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@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@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@xxxxxxxxxxxxxx/
>>> [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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