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References: <20200922210510.156220-1-lyude@redhat.com>
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From:   Ilia Mirkin <imirkin@alum.mit.edu>
Date:   Tue, 22 Sep 2020 17:10:03 -0400
Message-ID: <CAKb7UvhAb0wFd9Qi1FGJ=TAYZJ9DYXL6XXMfnG49xEO=a9TuYg@mail.gmail.com>
Subject: Re: [PATCH] drm/nouveau/kms/nv50-: Fix clock checking algorithm in nv50_dp_mode_valid()
To:     Lyude Paul <lyude@redhat.com>
Cc:     nouveau <nouveau@lists.freedesktop.org>,
        David Airlie <airlied@linux.ie>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:DRM DRIVER FOR NVIDIA GEFORCE/QUADRO GPUS" 
        <dri-devel@lists.freedesktop.org>, Ben Skeggs <bskeggs@redhat.com>
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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
>
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