Re: [igt-dev] [PATCH i-g-t 0/5] Fix mode selection for 2x tests

[Imre Deak <imre.deak@intel.com>]

On Wed, Apr 14, 2021 at 05:24:40PM +0200, Daniel Vetter wrote:
> On Mon, Apr 12, 2021 at 03:51:40AM +0000, Modem, Bhanuprakash wrote:
> > > From: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > Sent: Saturday, April 10, 2021 1:48 AM
> > > To: Modem, Bhanuprakash <bhanuprakash.modem@intel.com>
> > > Cc: IGT development <igt-dev@lists.freedesktop.org>
> > > Subject: Re: [igt-dev] [PATCH i-g-t 0/5] Fix mode selection for 2x tests
> > > 
> > > On Fri, Apr 9, 2021 at 1:39 PM Bhanuprakash Modem
> > > <bhanuprakash.modem@intel.com> wrote:
> > > >
> > > > When two monitors connected through MST, the second monitor also
> > > > tries to use the same mode. So two such modes may not fit into the
> > > > link bandwidth.
> > > >
> > > > This series will find a combination of modes that fit into the BW.
> > > >
> > > > By parsing the PATH property, we can add a link_group_id field to
> > > > igt_output_t to identify all connectors that shares the BW of the
> > > > same link.
> > > >
> > > > Parsing the PATH property consists of finding all connectors with
> > > > the same X prefix in the X-Y[-Z...] path property format and assigning
> > > > a unique non-zero link_group_id to all such connectors. Connectors
> > > > without a PATH property are not an MST output, so their link_group_id
> > > > can be left at 0.
> > > >
> > > > IGT core helper would override the mode on all connectors that will
> > > > be modeset by the next igt_display_commit() call in the test. These
> > > > are all the connectors in igt_display_t that have a pending_pipe set
> > > > by the test up to the point of calling this helper.
> > > >
> > > > Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
> > > 
> > > Uh this is really not how kms is supposed to work. There are _tons_ of
> > > reasons why 2 crtc at the same time wont work, mst bw constraint is
> > > just one.
> > > 
> > > If you want to fix this, this should be fixed with atomic TEST_ONLY
> > In fact, we are doing the same in this series.
> > 
> > By parsing the PATH connector prop, igt_output_refresh() will update the
> > link_group_id field for each connector [1]. 
> > 
> > Each individual (Nx)-test will identify the connectors those are sharing
> > the MST bw (by reading the link_group_id field in igt_output_t), and call
> > the helper to find the suitable modes [2].
> > 
> > A helper function iterates through those N output/mode combinations. And
> > find the combination using the most BW by ATOMIC_TEST_ONLY and returned
> > to the test [3].
> > 
> > Am I missing anything?
> 
> Using TEST_ONLY sounds good. Trying to do clever filtering with PATH
> property before you call TEST_ONLY is not good. You should check with
> TEST_ONLY in general, not just when the path property indicates that the
> dp output is shared.

I think it would still make sense to find the working config first on
outputs sharing a link bandwidth with TEST_ONLY and only then find the
config with all required outputs included in the TEST_ONLY commit,
starting with the modes found for outpus sharing a link. This is the way
you could find the maximum resolution that can be used on each output.

> -Daniel
> 
> > 
> > [1]: https://patchwork.freedesktop.org/patch/427718
> > [2]: https://patchwork.freedesktop.org/patch/427720
> > [3]: https://patchwork.freedesktop.org/patch/427719
> >  N : 2,3,4,...
> > 
> > -Bhanu
> > > mode to figure out what works and what doesn't. Not by trying to
> > > re-implement the kernel's atomic_check configuration validation,
> > > because you just can't do that.
> > > 
> > > So nack on architectural reasons on this approach.
> > > -Daniel
> > > 
> > > >
> > > > Bhanuprakash Modem (5):
> > > >   lib/igt_kms: Add a support to read PATH connector property
> > > >   lib/igt_kms: Identify outputs that shares link bandwidth
> > > >   lib/igt_kms: helper to override the mode on all connectors
> > > >   tests/kms_frontbuffer_tracking: Fix mode selection for 2x tests
> > > >   tests/kms_cursor_legacy: Fix mode selection for 2x tests
> > > >
> > > >  lib/igt_kms.c                    | 71 ++++++++++++++++++++++++++++++++
> > > >  lib/igt_kms.h                    |  3 ++
> > > >  tests/kms_cursor_legacy.c        | 53 ++++++++++++++++++++++--
> > > >  tests/kms_frontbuffer_tracking.c | 35 ++++++++++++++++
> > > >  4 files changed, 159 insertions(+), 3 deletions(-)
> > > >
> > > > --
> > > > 2.20.1
> > > >
> > > > _______________________________________________
> > > > igt-dev mailing list
> > > > igt-dev@lists.freedesktop.org
> > > > https://lists.freedesktop.org/mailman/listinfo/igt-dev
> > > 
> > > 
> > > 
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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