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SubjectRe: [PATCH] drm/nouveau/kms/nv50-: Fix clock checking algorithm in nv50_dp_mode_valid()
Can we use 6bpc on arbitrary DP monitors, or is there a capability for
it? Maybe only use 6bpc if display_info.bpc == 6 and otherwise use 8?

On Tue, Sep 22, 2020 at 5:06 PM Lyude Paul <lyude@redhat.com> wrote:
>
> While I thought I had this correct (since it actually did reject modes
> like I expected during testing), Ville Syrjala from Intel pointed out
> that the logic here isn't correct. max_clock refers to the max symbol
> rate supported by the encoder, so limiting clock to ds_clock using max()
> doesn't make sense. Additionally, we want to check against 6bpc for the
> time being since that's the minimum possible bpc here, not the reported
> bpc from the connector. See:
>
> https://lists.freedesktop.org/archives/dri-devel/2020-September/280276.html
>
> For more info.
>
> So, let's rewrite this using Ville's advice.
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Fixes: 409d38139b42 ("drm/nouveau/kms/nv50-: Use downstream DP clock limits for mode validation")
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Ben Skeggs <bskeggs@redhat.com>
> ---
> drivers/gpu/drm/nouveau/nouveau_dp.c | 23 +++++++++++++----------
> 1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_dp.c b/drivers/gpu/drm/nouveau/nouveau_dp.c
> index 7b640e05bd4cd..24c81e423d349 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_dp.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_dp.c
> @@ -231,23 +231,26 @@ nv50_dp_mode_valid(struct drm_connector *connector,
> const struct drm_display_mode *mode,
> unsigned *out_clock)
> {
> - const unsigned min_clock = 25000;
> - unsigned max_clock, ds_clock, clock;
> + const unsigned int min_clock = 25000;
> + unsigned int max_clock, ds_clock, clock;
> + const u8 bpp = 18; /* 6 bpc */
> enum drm_mode_status ret;
>
> if (mode->flags & DRM_MODE_FLAG_INTERLACE && !outp->caps.dp_interlace)
> return MODE_NO_INTERLACE;
>
> max_clock = outp->dp.link_nr * outp->dp.link_bw;
> - ds_clock = drm_dp_downstream_max_dotclock(outp->dp.dpcd,
> - outp->dp.downstream_ports);
> - if (ds_clock)
> - max_clock = min(max_clock, ds_clock);
> -
> - clock = mode->clock * (connector->display_info.bpc * 3) / 10;
> - ret = nouveau_conn_mode_clock_valid(mode, min_clock, max_clock,
> - &clock);
> + clock = mode->clock * bpp / 8;
> + if (clock > max_clock)
> + return MODE_CLOCK_HIGH;
> +
> + ds_clock = drm_dp_downstream_max_dotclock(outp->dp.dpcd, outp->dp.downstream_ports);
> + if (ds_clock && mode->clock > ds_clock)
> + return MODE_CLOCK_HIGH;
> +
> + ret = nouveau_conn_mode_clock_valid(mode, min_clock, max_clock, &clock);
> if (out_clock)
> *out_clock = clock;
> +
> return ret;
> }
> --
> 2.26.2
>
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

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