On Wed, Jun 1, 2022 at 2:23 PM Douglas Anderson <dianders@xxxxxxxxxxxx> 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. Since we have no true information about the connected > display, these modes are essentially guesses but better than nothing. > At the moment, none of the modes returned is marked as preferred, but > the modes are sorted such that the higher resolution modes are listed > first. > > When userspace sees these modes presented by the kernel it needs to > figure out which one to pick. At least one userspace, ChromeOS [1] > seems to use the rules (which seem pretty reasonable): > 1. Try to pick the first mode marked as preferred. > 2. Try to pick the mode which matches the first detailed timing > descriptor in the EDID. > 3. If no modes were marked as preferred then pick the first mode. > > Unfortunately, userspace's rules combined with what the kernel is > doing causes us to fail section 4.2.2.6 (EDID Corruption Detection) of > the DP 1.4a Link CTS. That test case says that, while it's OK to allow > some implementation-specific fall-back modes if the EDID is bad that > userspace should _default_ to 640x480. > > Let's fix this by marking 640x480 as default for DP in the no-EDID > case. > > NOTES: > - In the discussion around v3 of this patch [2] there was talk about > solving this in userspace and I even implemented a patch that would > have solved this for ChromeOS, but then the discussion turned back > to solving this in the kernel. > - Also in the discussion of v3 [2] it was requested to limit this > 83;40900;0c change to just DP since folks were worried that it would break some > subtle corner case on VGA or HDMI. > > [1] https://source.chromium.org/chromium/chromium/src/+/a051f741d0a15caff2251301efe081c30e0f4a96:ui/ozone/platform/drm/common/drm_util.cc;l=488 > [2] https://lore.kernel.org/r/20220513130533.v3.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > Reviewed-by: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> Reviewed-by: Sean Paul <seanpaul@xxxxxxxxxxxx> > > --- > I put Abhinav's Reviewed-by tag from v2 here since this is nearly the > same as v2. Hope this is OK. > > Changes in v4: > - Code is back to v2, but limit to just DP. > - Beefed up the commit message. > > Changes in v3: > - Don't set preferred, just disable the sort. > > 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 425f56280d51..75a71649b64d 100644 > --- a/drivers/gpu/drm/drm_probe_helper.c > +++ b/drivers/gpu/drm/drm_probe_helper.c > @@ -569,8 +569,17 @@ 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); > + > + /* > + * Section 4.2.2.6 (EDID Corruption Detection) of the DP 1.4a > + * Link CTS specifies that 640x480 (the official "failsafe" > + * mode) needs to be the default if there's no EDID. > + */ > + if (connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) > + drm_set_preferred_mode(connector, 640, 480); > + } > count += drm_helper_probe_add_cmdline_mode(connector); > if (count != 0) { > ret = __drm_helper_update_and_validate(connector, maxX, maxY, &ctx); > -- > 2.36.1.255.ge46751e96f-goog >
