linux-kernel - Re: [Nouveau] [PATCH 3/3] drm/nouveau/kms/nv50-: Limit MST BPC to 8

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Date:   Mon, 18 Nov 2019 14:36:14 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Lyude Paul <lyude@...hat.com>
Cc:     nouveau@...ts.freedesktop.org, Sam Ravnborg <sam@...nborg.org>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Jerry Zuo <Jerry.Zuo@....com>,
        Sean Paul <seanpaul@...omium.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...hat.com>,
        Harry Wentland <harry.wentland@....com>,
        Juston Li <juston.li@...el.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [Nouveau] [PATCH 3/3] drm/nouveau/kms/nv50-: Limit MST BPC to 8

On Fri, Nov 15, 2019 at 04:07:20PM -0500, Lyude Paul wrote:
> Noticed this while working on some unrelated CRC stuff. Currently,
> userspace has very little support for BPCs higher than 8. While this
> doesn't matter for most things, on MST topologies we need to be careful
> about ensuring that we do our best to make any given display
> configuration fit within the bandwidth restraints of the topology, since
> otherwise less people's monitor configurations will work.
> 
> Allowing for BPC settings higher than 8 dramatically increases the
> required bandwidth for displays in most configurations, and consequently
> makes it a lot less likely that said display configurations will pass
> the atomic check.
> 
> In the future we want to fix this correctly by making it so that we
> adjust the bpp for each display in a topology to be as high as possible,
> while making sure to lower the bpp of each display in the event that we
> run out of bandwidth and need to rerun our atomic check. But for now,
> follow the behavior that both i915 and amdgpu are sticking to.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Fixes: 232c9eec417a ("drm/nouveau: Use atomic VCPI helpers for MST")
> Cc: Ben Skeggs <bskeggs@...hat.com>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> Cc: David Airlie <airlied@...hat.com>
> Cc: Jerry Zuo <Jerry.Zuo@....com>
> Cc: Harry Wentland <harry.wentland@....com>
> Cc: Juston Li <juston.li@...el.com>
> Cc: Sam Ravnborg <sam@...nborg.org>
> Cc: Sean Paul <seanpaul@...omium.org>
> Cc: <stable@...r.kernel.org> # v5.1+
> ---
>  drivers/gpu/drm/nouveau/dispnv50/disp.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)

Reviewed-by: Thierry Reding <treding@...dia.com>

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