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    Subject[PATCH 5.13 195/266] drm/dp: Handle zeroed port counts in drm_dp_read_downstream_info()
    Date
    From: Lyude Paul <lyude@redhat.com>

    commit 205bb69a90363541a634a662a599fddb95956524 upstream.

    While the DP specification isn't entirely clear on if this should be
    allowed or not, some branch devices report having downstream ports present
    while also reporting a downstream port count of 0. So to avoid breaking
    those devices, we need to handle this in drm_dp_read_downstream_info().

    So, to do this we assume there's no downstream port info when the
    downstream port count is 0.

    Signed-off-by: Lyude Paul <lyude@redhat.com>
    Tested-by: Jérôme de Bretagne <jerome.debretagne@gmail.com>
    Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3416
    Fixes: 3d3721ccb18a ("drm/i915/dp: Extract drm_dp_read_downstream_info()")
    Cc: <stable@vger.kernel.org> # v5.10+
    Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20210430223428.10514-1-lyude@redhat.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/gpu/drm/drm_dp_helper.c | 7 +++++++
    1 file changed, 7 insertions(+)

    --- a/drivers/gpu/drm/drm_dp_helper.c
    +++ b/drivers/gpu/drm/drm_dp_helper.c
    @@ -679,7 +679,14 @@ int drm_dp_read_downstream_info(struct d
    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
    return 0;

    + /* Some branches advertise having 0 downstream ports, despite also advertising they have a
    + * downstream port present. The DP spec isn't clear on if this is allowed or not, but since
    + * some branches do it we need to handle it regardless.
    + */
    len = drm_dp_downstream_port_count(dpcd);
    + if (!len)
    + return 0;
    +
    if (dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE)
    len *= 4;


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