Date:   Fri, 14 Oct 2016 13:39:15 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Archit Taneja <architt@...eaurora.org>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, liviu.dudau@....com,
        robdclark@...il.com, hverkuil@...all.nl, eric@...olt.net,
        ville.syrjala@...ux.intel.com, daniel@...ll.ch
Subject: Re: [RFC PATCH 00/11] Introduce writeback connectors

Hi Archit,

On Fri, Oct 14, 2016 at 04:20:14PM +0530, Archit Taneja wrote:
>Hi Brian,
>
>On 10/11/2016 08:23 PM, Brian Starkey wrote:
>>Hi,
>>
>>This RFC series introduces a new connector type:
>> DRM_MODE_CONNECTOR_WRITEBACK
>>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.
>>
>>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.
>>
>>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).
>> * 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.
>> * Documentation. Probably some portion of this cover letter needs to
>>   make it into Documentation/
>> * 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.
>>
>>See Also:
>>---------
>>[1] https://lists.freedesktop.org/archives/dri-devel/2016-July/113197.html
>>[2] 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.
>
>Thanks for working on this! Some points below.
>
>- Writeback hardware generally allows us to specify the region within
>the framebuffer we want to write to. It's analogous to the SRC_X/Y/W/H
>plane properties. We could have similar props for the writeback
>connectors, and maybe set them to the FB_ID dimensions if they aren't
>configured by userspace.
>
>- Besides the above property, writeback hardware can have provisions
>for scaling, color space conversion and rotation. This would mean that
>we'd eventually add more writeback specific props/params in
>drm_connector/drm_connector_state. Would we be okay adding more such
>props for connectors?

I've wondered the same thing about bloating non-writeback connectors
with writeback-specific stuff. If it does become significant, maybe
we should subclass drm_connector and add a drm_writeback_state pointer
to drm_connector_state.

Ville touched on scaling support previously, suggesting adding a
fixed_mode property (for all types of connectors) - on writeback this
would represent scaling the framebuffer, and on normal connectors it
could control output scaling (like panel-fitting).

Certainly destination coords, color-space converstion etc. are things
that are worth adding, but IMO I'd rather keep this initial
implementation small so we can enable the basic case right away. For
the most part, the additional things are "just properties" which
should be easily added later without impacting the overall interface.

Cheers,
Brian
>
>Thanks,
>Archit
>
>>
>>Thanks,
>>
>>-Brian
>>
>>---
>>
>>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
>>
>
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