Re: [RFC PATCH 00/11] Introduce writeback connectors
From: Daniel Vetter
Date: Tue Oct 11 2016 - 11:44:10 EST
On Tue, Oct 11, 2016 at 03:53:57PM +0100, Brian Starkey wrote:
> This RFC series introduces a new connector type:
> It is a follow-on from a previous discussion: 
> Writeback connectors are used to expose the memory writeback engines
> found in some display controllers, which can write a CRTC's
> composition result to a memory buffer.
> This is useful e.g. for testing, screen-recording, screenshots,
> wireless display, display cloning, memory-to-memory composition.
> Patches 1-7 include the core framework changes required, and patches
> 8-11 implement a writeback connector for the Mali-DP writeback engine.
> The Mali-DP patches depend on this other series: .
> The connector is given the FB_ID property for the output framebuffer,
> and two new read-only properties: PIXEL_FORMATS and
> PIXEL_FORMATS_SIZE, which expose the supported framebuffer pixel
> formats of the engine.
> The EDID property is not exposed for writeback connectors.
> Writeback connector usage:
> Due to connector routing changes being treated as "full modeset"
> operations, any client which wishes to use a writeback connector
> should include the connector in every modeset. The writeback will not
> actually become active until a framebuffer is attached.
Erhm, this is just the default, drivers can override this. And we could
change the atomic helpers to not mark a modeset as a modeset if the
connector that changed is a writeback one.
> The writeback itself is enabled by attaching a framebuffer to the
> FB_ID property of the connector. The driver must then ensure that the
> CRTC content of that atomic commit is written into the framebuffer.
> The writeback works in a one-shot mode with each atomic commit. This
> prevents the same content from being written multiple times.
> In some cases (front-buffer rendering) there might be a desire for
> continuous operation - I think a property could be added later for
> this kind of control.
> Writeback can be disabled by setting FB_ID to zero.
This seems to contradict itself: If it's one-shot, there's no need to
disable it - it will auto-disable.
In other cases where we write a property as a one-shot thing (fences for
android). In that case when you read that property it's always 0 (well, -1
for fences since file descriptor). That also avoids the issues when
userspace unconditionally saves/restores all properties (this is needed
for generic compositor switching).
I think a better behaviour would be to do the same trick, with FB_ID on
the connector always returning 0 as the current value. That encodes the
one-shot behaviour directly.
For one-shot vs continuous: Maybe we want to simply have a separate
writeback property for continues, e.g. FB_WRITEBACK_ONE_SHOT_ID and
> Known issues:
> * I'm not sure what "DPMS" should mean for writeback connectors.
> It could be used to disable writeback (even when a framebuffer is
> attached), or it could be hidden entirely (which would break the
> legacy DPMS call for writeback connectors).
dpms is legacy, in atomic land the only thing you have is "ACTIVE" on the
crtc. it disables everything, i.e. also writeback.
> * With Daniel's recent re-iteration of the userspace API rules, I
> fully expect to provide some userspace code to support this. The
> question is what, and where? We want to use writeback for testing,
> so perhaps some tests in igt is suitable.
Hm, testing would be better as a debugfs interface, but I understand the
appeal of doing this with atomic (since semantics fit so well). Another
use-case of this is compositing, but if the main goal is igt and testing,
I think integration into igt crc based testcases is a perfectly fine
> * Documentation. Probably some portion of this cover letter needs to
> make it into Documentation/
Yeah, an overview DOC: section in a separate source file (with all the the
infrastructure work) would be great - aka needed from my pov ;-)
> * Synchronisation. Our hardware will finish the writeback by the next
> vsync. I've not implemented fence support here, but it would be an
> obvious addition.
Probably just want an additional WRITEBACK_FENCE_ID property to signal
completion. Some hw definitely will take longer to write back than just a
vblank. But we can delay that until it's needed.
> See Also:
>  https://lists.freedesktop.org/archives/dri-devel/2016-July/113197.html
>  https://lists.freedesktop.org/archives/dri-devel/2016-October/120486.html
> I welcome any comments, especially if this approach does/doesn't fit
> well with anyone else's hardware.
> Brian Starkey (10):
> drm: add writeback connector type
> drm/fb-helper: skip writeback connectors
> drm: extract CRTC/plane disable from drm_framebuffer_remove
> drm: add __drm_framebuffer_remove_atomic
> drm: add fb to connector state
> drm: expose fb_id property for writeback connectors
> drm: add writeback-connector pixel format properties
> drm: mali-dp: rename malidp_input_format
> drm: mali-dp: add RGB writeback formats for DP550/DP650
> drm: mali-dp: add writeback connector
> Liviu Dudau (1):
> drm: mali-dp: Add support for writeback on DP550/DP650
> drivers/gpu/drm/arm/Makefile | 1 +
> drivers/gpu/drm/arm/malidp_crtc.c | 10 ++
> drivers/gpu/drm/arm/malidp_drv.c | 25 +++-
> drivers/gpu/drm/arm/malidp_drv.h | 5 +
> drivers/gpu/drm/arm/malidp_hw.c | 104 ++++++++++----
> drivers/gpu/drm/arm/malidp_hw.h | 27 +++-
> drivers/gpu/drm/arm/malidp_mw.c | 268 +++++++++++++++++++++++++++++++++++
> drivers/gpu/drm/arm/malidp_planes.c | 8 +-
> drivers/gpu/drm/arm/malidp_regs.h | 15 ++
> drivers/gpu/drm/drm_atomic.c | 40 ++++++
> drivers/gpu/drm/drm_atomic_helper.c | 4 +
> drivers/gpu/drm/drm_connector.c | 79 ++++++++++-
> drivers/gpu/drm/drm_crtc.c | 14 +-
> drivers/gpu/drm/drm_fb_helper.c | 4 +
> drivers/gpu/drm/drm_framebuffer.c | 249 ++++++++++++++++++++++++++++----
> drivers/gpu/drm/drm_ioctl.c | 7 +
> include/drm/drmP.h | 2 +
> include/drm/drm_atomic.h | 3 +
> include/drm/drm_connector.h | 15 ++
> include/drm/drm_crtc.h | 12 ++
> include/uapi/drm/drm.h | 10 ++
> include/uapi/drm/drm_mode.h | 1 +
> 22 files changed, 830 insertions(+), 73 deletions(-)
> create mode 100644 drivers/gpu/drm/arm/malidp_mw.c
Software Engineer, Intel Corporation