linux-kernel - Re: [PATCH 2/7] drm/msm/dsi: Pass bonded-DSI hdisplay/2 to DSC timing configuration

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Date: Sat, 20 Apr 2024 01:59:16 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Marijn Suijten <marijn.suijten@...ainline.org>
Cc: Rob Clark <robdclark@...il.com>, 
	Abhinav Kumar <quic_abhinavk@...cinc.com>, Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>, 
	Daniel Vetter <daniel@...ll.ch>, Archit Taneja <architt@...eaurora.org>, 
	Chandan Uddaraju <chandanu@...eaurora.org>, Vinod Koul <vkoul@...nel.org>, 
	Sravanthi Kollukuduru <skolluku@...eaurora.org>, linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	Jordan Crouse <jordan@...micpenguin.net>, Rajesh Yadav <ryadav@...eaurora.org>, 
	Jeykumar Sankaran <jsanka@...eaurora.org>, ~postmarketos/upstreaming@...ts.sr.ht, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, Konrad Dybcio <konrad.dybcio@...aro.org>, 
	Martin Botka <martin.botka@...ainline.org>, Jami Kettunen <jami.kettunen@...ainline.org>
Subject: Re: [PATCH 2/7] drm/msm/dsi: Pass bonded-DSI hdisplay/2 to DSC
 timing configuration

On Sat, Apr 20, 2024 at 12:18:39AM +0200, Marijn Suijten wrote:
> On 2024-04-17 14:58:25, Dmitry Baryshkov wrote:
> > On Wed, 17 Apr 2024 at 02:57, Marijn Suijten
> > <marijn.suijten@...ainline.org> wrote:
> > >
> > > When configuring the timing of DSI hosts (interfaces) in
> > > dsi_timing_setup() all values written to registers are taking bonded
> > > DSI into account by dividing the original mode width by 2 (half the
> > > data is sent over each of the two DSI hosts), but the full width
> > > instead of the interface width is passed as hdisplay parameter to
> > > dsi_update_dsc_timing().
> > >
> > > Currently only msm_dsc_get_slices_per_intf() is called within
> > > dsi_update_dsc_timing() with the `hdisplay` argument which clearly
> > > documents that it wants the width of a single interface (which, again,
> > > in bonded DSI mode is half the total width of the mode).  Thus pass the
> > > bonded-mode-adjusted hdisplay parameter into dsi_update_dsc_timing()
> > > otherwise all values written to registers by this function (i.e. the
> > > number of slices per interface or packet, and derived from this the EOL
> > > byte number) are twice too large.
> > >
> > > Inversely the panel driver is expected to only set the slice width and
> > > number of slices for half the panel, i.e. what will be sent by each
> > > host individually, rather than fixing that up like hdisplay here.
> > >
> > > Fixes: 08802f515c3c ("drm/msm/dsi: Add support for DSC configuration")
> > > Signed-off-by: Marijn Suijten <marijn.suijten@...ainline.org>
> > > ---
> > >  drivers/gpu/drm/msm/dsi/dsi_host.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> 
> Thanks, it seems this patch has already been picked up for 6.10 [1] to test at
> least, but I'd advise you to drop it until I resend it in v2, as it no longer
> performs as written in the title.

Ok, dropping.

> 
> When I wrote this patch in in June 2023, commit efcbd6f9cdeb ("drm/msm/
> dsi: Enable widebus for DSI") from August 2023 wasn't there yet.  That patch
> updates hdisplay (because it is unused after that point) with the number
> of compressed bytes to be sent over each interface, which is effectively
> hdisplay (based on slice_count * slice_width, so as explained in the commit
> message that corresponds to half the panel width), divided by a compression
> ratio of 3 or 6 depending on widebus, thus passing a way too low value into
> dsi_update_dsc_timing().
> 
> As a result this patch regresses the DSC panel on my SM8150 Sony Xperia 1, and
> likely also explains why it was quite hard to get the porches "just right" on
> the Xperia 1 III with its dual-DSI dual-DSC 4k@...Hz panel (that these patches
> are specifically for).
> 
> I'm still thinking of how to best fix that: probably introducing a new separate
> local variable, though dsi_update_dsc_timing() only uses it to calculate
> the number of slices per interface, which again as written in the commit
> description, is currently required to already be for one interface (in other
> words, the Xperia 1 with only a single intf sets slice_count=2, but the Xperia 1
> III with 2 bonded DSI interfaces sets slice_count=1).  Which means that this is
> always equivalent to slice_per_intf = dsc->slice_count.
> 
> Let me know which approach is preferred.
> 
> - Marijn
> 
> [1]: https://gitlab.freedesktop.org/drm/msm/-/merge_requests/110

-- 
With best wishes
Dmitry