On Wed, Sep 25, 2024 at 07:25:34PM -0500, Benjamin Hoefs wrote: > Hello, > > I am using a Dell WD19TB dock with a 3440x1440 monitor. Using it at > 100Hz used to work but recently I tried it again and discovered it no longer > did, specifically the modeset seems to silently fail with no error message in > userspace utilities like kscreen-doctor and xrandr and no output in dmesg. > I found the problematic commit using git bisect to be > 55eaef164174480df6827edeac15620f3cbcd52b "Handle the Synaptics HBlank > expansion quirk". > > I found the issue to be the hblank_expasion_quirk_needs_dsc function which uses > the following comparison in the current kernel tree: > > if (mode_hblank_period_ns(adjusted_mode) > hblank_limit) > return false; > > with hblank_limit being earlier set as > > int hblank_limit = is_uhbr_sink ? 500 : 300; > > However, my monitor's HBLANK period in the 3440x1440@100Hz mode is > exactly 300 ns as verified by this printk immediately before the > problematic comparison. > > printk(KERN_INFO "Hello, kernel world! %i\n", > mode_hblank_period_ns(adjusted_mode)); > [ 38.429839] Hello, kernel world! 300 > > With the attached change the modeset works as expected at 100Hz. Would it be > acceptable to modify the comparison from > to >= here? > > I'll do my best to provide any additional details you may need although > that printk and '=' sign is the only kernel code I've written, so my best may > not be great :). > > Signed-off-by: Benjamin D. Hoefs <bendhoefs@xxxxxxxxx> > --- > drivers/gpu/drm/i915/display/intel_dp_mst.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c > index 15541932b809..052c5a67df93 100644 > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c > @@ -446,7 +446,7 @@ hblank_expansion_quirk_needs_dsc(const struct intel_connector *connector, > if (is_uhbr_sink && !drm_dp_is_uhbr_rate(limits->max_rate)) > return false; > > - if (mode_hblank_period_ns(adjusted_mode) > hblank_limit) > + if (mode_hblank_period_ns(adjusted_mode) >= hblank_limit) > return false; Disabling DSC this way could make another mode with the same hblank period not work. This mode would require DSC in any case if there is a link BW limitation, so would need to check why DSC is failing. Could you open a ticket at: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/new attaching a dmesg log booting with drm.debug=0x15e? Thanks, Imre > > return true; > -- > 2.46.2 >
