[PATCH 0/5] Unmappable DRM client buffers for fbdev emulation

[Thomas Zimmermann]

Hi

Am 03.07.19 um 21:27 schrieb Noralf Trønnes:
> 
> 
> Den 03.07.2019 10.32, skrev Thomas Zimmermann:
>> DRM client buffers are permanently mapped throughout their lifetime. This
>> prevents us from using generic framebuffer emulation for devices with
>> small dedicated video memory, such as ast or mgag200. With fb buffers
>> permanently mapped, such devices often won't have enougth space left to
>> display other content (e.g., X11).
>>
>> This patch set introduces unmappable DRM client buffers for framebuffer
>> emulation with shadow buffers. While the shadow buffer remains in system
>> memory permanently, the respective buffer object will only be mapped briefly
>> during updates from the shadow buffer. Hence, the driver can relocate he
>> buffer object among memory regions as needed.
>>
>> The default behoviour for DRM client buffers is still to be permanently
>> mapped.
>>
>> The patch set converts ast and mgag200 to generic framebuffer emulation
>> and removes a large amount of framebuffer code from these drivers. For
>> bochs, a problem was reported where the driver could not display the console
>> because it was pinned in system memory. [1] The patch set fixes this bug
>> by converting bochs to use the shadow fb.
>>
>> The patch set has been tested on ast and mga200 HW.
>>
> 
> I've been thinking, this enables dirty tracking in DRM userspace for
> these drivers since the dirty callback is now set on all framebuffers.
> Is this OK? Should we add a drm_fbdev_generic_shadow_setup() that sets a
> flag enabling the shadow buffer instead?

Fbdev emulation is special wrt framebuffer setup and there's no way to
distinguish a regular FB from the fbdev's FB. I've been trying
drm_fbdev_generic_shadow_setup(), but ended up duplicating
functionality. The problem was that we cannot get state-flag arguments
into drm_fb_helper_generic_probe().

There already is struct mode_config.prefer_shadow. It signals shadow FB
rendering to userspace. The easiest solution is to add
prefer_shadow_fbdev as well. If either flag is true, fbdev emulation
would enable shadow buffering.

I'm not sure if we should check for the dirty() callback at all.

Best regards
Thomas

> Really nice diffstat by the way :-)
> 
> Noralf.
> 
>> [1] https://lists.freedesktop.org/archives/dri-devel/2019-June/224423.html
>>
>> Thomas Zimmermann (5):
>>   drm/client: Support unmapping of DRM client buffers
>>   drm/fb-helper: Unmap BO for shadow-buffered framebuffer console
>>   drm/ast: Replace struct ast_fbdev with generic framebuffer emulation
>>   drm/bochs: Use shadow buffer for bochs framebuffer console
>>   drm/mgag200: Replace struct mga_fbdev with generic framebuffer
>>     emulation
>>
>>  drivers/gpu/drm/ast/Makefile           |   2 +-
>>  drivers/gpu/drm/ast/ast_drv.c          |  22 +-
>>  drivers/gpu/drm/ast/ast_drv.h          |  17 --
>>  drivers/gpu/drm/ast/ast_fb.c           | 341 -------------------------
>>  drivers/gpu/drm/ast/ast_main.c         |  30 ++-
>>  drivers/gpu/drm/ast/ast_mode.c         |  21 --
>>  drivers/gpu/drm/bochs/bochs_kms.c      |   2 +-
>>  drivers/gpu/drm/drm_client.c           |  71 ++++-
>>  drivers/gpu/drm/drm_fb_helper.c        |  14 +-
>>  drivers/gpu/drm/mgag200/Makefile       |   2 +-
>>  drivers/gpu/drm/mgag200/mgag200_drv.h  |  19 --
>>  drivers/gpu/drm/mgag200/mgag200_fb.c   | 309 ----------------------
>>  drivers/gpu/drm/mgag200/mgag200_main.c |  61 +++--
>>  drivers/gpu/drm/mgag200/mgag200_mode.c |  27 --
>>  include/drm/drm_client.h               |   3 +
>>  15 files changed, 154 insertions(+), 787 deletions(-)
>>  delete mode 100644 drivers/gpu/drm/ast/ast_fb.c
>>  delete mode 100644 drivers/gpu/drm/mgag200/mgag200_fb.c
>>
>> --
>> 2.21.0
>>
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-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah
HRB 21284 (AG Nürnberg)

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