Re: [PATCH v3 0/2] Add writeback scaling

[Liviu Dudau]

On Tue, Apr 23, 2019 at 02:24:11PM +0200, Daniel Vetter wrote:
> On Tue, Apr 23, 2019 at 12:48 PM Ben Davis <Ben.Davis@xxxxxxx> 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.

Is that they way it should work, though? I remember a few good years
back, before KMS, there were lots of issues with X server hanging and
not restoring the vt mode correctly. Should it not be that on getting
back control of a DRM master, an application sets it last known good
mode that it had, before it lost (or gave away) control?

> 
> 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.

Repeating just to make sure we're on the same page: we allow 0 as valid
range value, but writeback_dst_h/w ultimately gets updated before the
commit gets passed on to the driver to use crtc_state->mode.v/hdisplay
so as not to trigger any scaling. Correct?

Best regards,
Liviu


> -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

-- 
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