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    SubjectRe: [PATCH v2] drm/probe-helper: Default to 640x480 if no EDID
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    On 5/10/2022 1:51 PM, Douglas Anderson wrote:
    > If we're unable to read the EDID for a display because it's corrupt /
    > bogus / invalid then we'll add a set of standard modes for the
    > display. When userspace looks at these modes it doesn't really have a
    > good concept for which mode to pick and it'll likely pick the highest
    > resolution one by default. That's probably not ideal because the modes
    > were purely guesses on the part of the Linux kernel.
    >
    > Let's instead set 640x480 as the "preferred" mode when we have no EDID.
    >
    > Signed-off-by: Douglas Anderson <dianders@chromium.org>
    Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
    > ---
    > Note that this is the second of two related and similar-sounding but
    > different patches. See also ("drm/probe-helper: For DP, add 640x480 if
    > all other modes are bad") [1]. I'm hoping to land _both_ of the
    > patches since they address different issues. This patch addresses the
    > case of a corrupt EDID and having 640x480 be the default in the
    > "guessed" modes. The other patch handles the case where the EDID
    > _isn't_ corrupt but all the modes listed can't be made with the
    > existing situations. The two patches can land in either order.
    >
    > Also note that I didn't carry any Tested-by / Reviewed-by tags since
    > the patch is now quite different.
    >
    > [1] https://lore.kernel.org/r/20220510131309.v2.2.I4ac7f55aa446699f8c200a23c10463256f6f439f@changeid
    >
    > Changes in v2:
    > - Don't modify drm_add_modes_noedid() 'cause that'll break others
    > - Set 640x480 as preferred in drm_helper_probe_single_connector_modes()
    >
    > drivers/gpu/drm/drm_probe_helper.c | 11 ++++++++++-
    > 1 file changed, 10 insertions(+), 1 deletion(-)
    >
    > diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
    > index 682359512996..1fbb9a8c315c 100644
    > --- a/drivers/gpu/drm/drm_probe_helper.c
    > +++ b/drivers/gpu/drm/drm_probe_helper.c
    > @@ -516,8 +516,17 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
    > count = drm_add_override_edid_modes(connector);
    >
    > if (count == 0 && (connector->status == connector_status_connected ||
    > - connector->status == connector_status_unknown))
    > + connector->status == connector_status_unknown)) {
    > count = drm_add_modes_noedid(connector, 1024, 768);
    > +
    > + /*
    > + * Give userspace a hint that we don't have a lot of confidence
    > + * in these modes (we totally guessed) by marking 640x480 as
    > + * preferred. This is low clock rate and incredibly common as
    > + * a failsafe mode.
    > + */
    > + drm_set_preferred_mode(connector, 640, 480);
    > + }
    > count += drm_helper_probe_add_cmdline_mode(connector);
    > if (count == 0)
    > goto prune;

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