Re: [PATCH v2] drm/msm/dp: populate connector of struct dp_panel

[Stephen Boyd <swboyd@chromium.org>]

Quoting Kuogee Hsieh (2021-12-29 11:17:02)
> There is kernel crashed due to unable to handle kernel NULL
> pointer dereference of dp_panel->connector while running DP link
> layer compliance test case 4.2.2.6 (EDID Corruption Detection).

Can you explain how we get into that situation? Like

"We never assign struct dp_panel::connector, instead the connector is
stored in struct msm_dp::connector. When we run compliance testing test
case 4.2.2.6 dp_panel_handle_sink_request() won't have a valid edid set
in struct dp_panel::edid so we'll try to use the connectors
real_edid_checksum and hit a NULL pointer deref error because the
connector pointer is never assigned."

> This patch fixes this problem by populating connector of dp_panel.
>
> [drm:dp_panel_read_sink_caps] *ERROR* panel edid read failed
> Unable to handle kernel NULL pointer dereference at virtual address 00000000000006e1
> Mem abort info:
>   ESR = 0x96000006
>   EC = 0x25: DABT (current EL), IL = 32 bits
>   SET = 0, FnV = 0
>   EA = 0, S1PTW = 0
> Data abort info:
>   ISV = 0, ISS = 0x00000006
>   CM = 0, WnR = 0
> user pgtable: 4k pages, 39-bit VAs, pgdp=0000000115f25000
> [00000000000006e1] pgd=00000001174fe003, p4d=00000001174fe003, pud=00000001174fe003, pmd=0000000000000000
> Internal error: Oops: 96000006 [#1] PREEMPT SMP

This sort of stuff isn't really useful because it takes quite a few
lines to say "We hit a NULL pointer deref" which was already stated. I'd
rather have a clear description of what goes wrong and how setting the
pointer in msm_dp_modeset_init() fixes it.

> {...]
>
> Changes in V2:
> -- populate panel connector at msm_dp_modeset_init() instead of at dp_panel_read_sink_caps()
>
> Fixes: 7948fe12d47 ("drm/msm/dp: return correct edid checksum after corrupted edid checksum read")
> Signee-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
> ---
>  drivers/gpu/drm/msm/dp/dp_display.c | 25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index 3449d3f..c282bbf 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -1495,36 +1495,41 @@ void msm_dp_debugfs_init(struct msm_dp *dp_display, struct drm_minor *minor)
>         }
>  }
>
> -int msm_dp_modeset_init(struct msm_dp *dp_display, struct drm_device *dev,
> +int msm_dp_modeset_init(struct msm_dp *dp, struct drm_device *dev,
>                         struct drm_encoder *encoder)
>  {
>         struct msm_drm_private *priv;
> +       struct dp_display_private *dp_display;
>         int ret;
>
> -       if (WARN_ON(!encoder) || WARN_ON(!dp_display) || WARN_ON(!dev))
> +       if (WARN_ON(!encoder) || WARN_ON(!dp) || WARN_ON(!dev))
>                 return -EINVAL;
>
>         priv = dev->dev_private;
> -       dp_display->drm_dev = dev;
> +       dp->drm_dev = dev;
> +
> +       dp_display = container_of(dp, struct dp_display_private, dp_display);
>
> -       ret = dp_display_request_irq(dp_display);
> +       ret = dp_display_request_irq(dp);
>         if (ret) {
>                 DRM_ERROR("request_irq failed, ret=%d\n", ret);
>                 return ret;
>         }
>
> -       dp_display->encoder = encoder;
> +       dp->encoder = encoder;
>
> -       dp_display->connector = dp_drm_connector_init(dp_display);
> -       if (IS_ERR(dp_display->connector)) {
> -               ret = PTR_ERR(dp_display->connector);
> +       dp->connector = dp_drm_connector_init(dp);
> +       if (IS_ERR(dp->connector)) {
> +               ret = PTR_ERR(dp->connector);
>                 DRM_DEV_ERROR(dev->dev,
>                         "failed to create dp connector: %d\n", ret);
> -               dp_display->connector = NULL;
> +               dp->connector = NULL;
>                 return ret;
>         }
>
> -       priv->connectors[priv->num_connectors++] = dp_display->connector;
> +       dp_display->panel->connector = dp->connector;

This is the one line that matters I think? Can we reach the connector
for the dp device without going through the panel in
dp_panel_handle_sink_request()? That would reduce the number of struct
elements if possible.

> +
> +       priv->connectors[priv->num_connectors++] = dp->connector;

Can we not rename all the local variables in this patch and do it later
or never? Reading this patch takes a long time because we have to make
sure nothing has actually changed with the rename of 'dp_display' to
'dp'.

>         return 0;
>  }
>
> --
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