Re: [RFC PATCH 00/11] Introduce writeback connectors
From: Daniel Vetter
Date: Tue Oct 11 2016 - 13:02:31 EST
On Tue, Oct 11, 2016 at 6:43 PM, Brian Starkey <brian.starkey@xxxxxxx> wrote:
> Hi Daniel,
> Firstly thanks very much for having a look.
> On Tue, Oct 11, 2016 at 05:43:59PM +0200, Daniel Vetter wrote:
>> 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.
> Hmm, maybe. I don't think it's ideal - the driver would need to
> re-implement drm_atomic_helper_check_modeset, which is quite a chunk
> of code (along with exposing update_connector_routing, mode_fixup,
> maybe others), and even after that it would have to lie and set
> crtc_state->connectors_changed to false so that
> drm_crtc_needs_modeset returns false to drm_atomic_check_only.
You only need to update the property in your encoders's ->atomic_check
function. No need for more, and I think being consistent with
computing when you need a modeset is really a crucial part of the
atomic ioctl that we should imo try to implement correctly as much as
> I tried to keep special-casing of writeback connectors in the core to
> a bare minimum, because I think it will quickly get messy and fragile
Please always make the disdinction between core and shared drm
helpers. Special cases in core == probably not good. Special cases in
helpers == perfectly fine imo.
> Honestly, I don't see modesetting the writeback connectors at
> start-of-day as a big problem.
It's inconsistent. Claiming it needs a modeset when it doesn't isn't
great. Making that more discoverable to userspace is the entire point
of atomic. And there might be hw where reconfiguring for writeback
might need a full modeset.
>>> 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.
> I should have explained one-shot more clearly. What I mean is, one
> drmModeAtomicCommit == one write to memory. This is as opposed to
> writing the same thing to memory every vsync until it is stopped
> (which our HW is capable of doing).
> A subsequent drmModeAtomicCommit which doesn't touch the writeback FB_ID
> will write (again - but with whatever scene updates) to the same
> This continues for every drmModeAtomicCommit until FB_ID is set to
> zero - to disable writing - or changed to a different framebuffer, in
> which case we write to the new one.
> IMO this behaviour makes sense in the context of the rest of Atomic,
> and as the FB_ID is indeed persistent across atomic commits, I think
> it should be read-able.
tbh I don't like that, I think it'd be better to make this truly
one-shot. Otherwise we'll have real fun problems with hw where the
writeback can take longer than a vblank (it happens ...). So one-shot,
with auto-clearing to NULL/0 is imo the right approach.
>> 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.
> So removing the DPMS property is a viable option for writeback connectors in
> your opinion?
Nah, that's part of the abi now. But atomic internally remaps it to
"ACTIVE", in short you don't need to care (as long as you fill out the
dpms hook with the provided helper. drm_writeback_connector_init
should probably do that).
>>> * 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 ;-)
> Sure, I'll a look at splitting into a drm_writeback.c
>>> * 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:
>>> 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
>> Daniel Vetter
>> Software Engineer, Intel Corporation
> dri-devel mailing list
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch