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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Tue, 23 Apr 2019 14:24:11 +0200
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Subject: Re: [PATCH v3 0/2] Add writeback scaling
To:     Ben Davis <Ben.Davis@arm.com>
Cc:     "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        nd <nd@arm.com>, Liviu Dudau <Liviu.Dudau@arm.com>,
        Brian Starkey <Brian.Starkey@arm.com>,
        "airlied@linux.ie" <airlied@linux.ie>,
        "maarten.lankhorst@linux.intel.com" 
        <maarten.lankhorst@linux.intel.com>,
        "maxime.ripard@bootlin.com" <maxime.ripard@bootlin.com>,
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On Tue, Apr 23, 2019 at 12:48 PM Ben Davis <Ben.Davis@arm.com> wrote:
>
> Add support for scaling on writeback. To do this add writeback_dest_w
> and writeback_dest_h as writeback connector properties to specify the
> desired output dimensions.
> Then implement downscaling on writeback for Malidp-550 and Malidp-650
> (upscaling on writeback is not supported on these devices).
>
> v2: Use 0 as default for writeback_w,h and so update range to use 1 as
>     minimum.

Hm I missed that, I don't think that's good, since it prevents
userspace from blindly writing back the properties it's read. We've
tried hard to avoid that, since we're suggesting compositor can take a
snapshot of all kms properties (including the ones they don't
understand) and restore that on vt switching. Hence stuff like fence
fds returning -1, and accepting -1 as NULL to make this work.

tldr; I think range needs to include 0, and we need make that a
special thing, maybe enforced with a
drm_connector_state_compute_writeback_dst_h/w, which takes
crtc_state->mode.v/hdisplay into account if the writeback_dst_h/w is
0.
-Daniel

>
> v3: Rename properties to specify they are destination width/height.
>     Make sure the values from the properties are passed to
>     enable_memwrite rather than the framebuffer dimensions
>
> Ben Davis (2):
>   drm: Add writeback_dest_w,h properties
>   drm/malidp: Enable writeback scaling
>
>  drivers/gpu/drm/arm/malidp_crtc.c | 49 +++++++++++---------
>  drivers/gpu/drm/arm/malidp_crtc.h | 12 +++++
>  drivers/gpu/drm/arm/malidp_drv.c  | 10 +++--
>  drivers/gpu/drm/arm/malidp_hw.c   | 45 +++++++++++++------
>  drivers/gpu/drm/arm/malidp_hw.h   | 19 ++++++--
>  drivers/gpu/drm/arm/malidp_mw.c   | 94 ++++++++++++++++++++++++++++++---------
>  drivers/gpu/drm/arm/malidp_regs.h |  1 +
>  drivers/gpu/drm/drm_atomic_uapi.c |  8 ++++
>  drivers/gpu/drm/drm_writeback.c   | 30 +++++++++++++
>  include/drm/drm_connector.h       |  4 ++
>  include/drm/drm_mode_config.h     | 10 +++++
>  11 files changed, 220 insertions(+), 62 deletions(-)
>  create mode 100644 drivers/gpu/drm/arm/malidp_crtc.h
>
> --
> 2.7.4
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch