Re: [RFC PATCH 00/11] Introduce writeback connectors

From: Brian Starkey
Date: Tue Oct 11 2016 - 15:47:11 EST


On Tue, Oct 11, 2016 at 07:01:33PM +0200, Daniel Vetter wrote:
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: [1]

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: [2].

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

Sorry I really don't follow. Which property? CRTC_ID?

Userspace changing CRTC_ID will change connector_state->crtc (before
we even get to a driver callback).

After that, drm_atomic_helper_check_modeset derives connectors_changed
based on the ->crtc pointers.

After that, my encoder ->atomic_check *could* clear
connectors_changed (or I could achieve the same thing by wrapping
drm_atomic_helper_check), but it seems wrong to do so, considering
that the connector routing *has* changed.

If you think changing CRTC_ID shouldn't require a full modeset, I'd
rather give drivers a ->needs_modeset callback to override the default
drm_atomic_crtc_needs_modeset behaviour, instead of "tricking" it into
returning false.

I can imagine some hardware will need a full modeset to changed the
writeback CRTC binding anyway.

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.

I'm a little confused - what bit exactly is inconsistent?

My implementation here is consistent with other connectors.
Updating the writeback connector CRTC_ID property requires a full
modeset, the same as other connectors.

Changing the FB_ID does *not* require a full modeset, because our
hardware has no such restriction. This is analogous to updating the
FB_ID on our planes, and is consistent with the other instances of the
FB_ID property.

If there is hardware which does have a restriction on changing FB_ID, I think that driver must be responsible for handling it in the same
way as drivers which can't handle plane updates without a full

Are you saying that because setting CRTC_ID on Mali-DP is a no-op, it
shouldn't require a full modeset? I'd rather somehow hard-code the
CRTC_ID for our writeback connector to have it always attached to
the CRTC in that case.

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.

That's an interesting point about hardware which won't finish within
one frame; but I don't see how "true one-shot" helps.

What's the expected behaviour if userspace makes a new atomic commit
with a writeback framebuffer whilst a previous writeback is ongoing?

In both cases, you either need to block or fail the commit - whether
the framebuffer gets removed when it's done is immaterial.

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).

A connector can still be DPMS-ed individually, so a CRTC can be
"ACTIVE", attached to an "OFF" writeback connector, and the writeback
connector would still be able to actively write to memory.

I'm OK with that, and it's what I already implemented, but I thought
that userspace might reasonably expect a writeback connector with DPMS
set to "OFF" to be completely inert.


Cheers, Daniel

* 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

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Daniel Vetter
Software Engineer, Intel Corporation
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