[PATCH] drm/i915/display: Allow tighter hblank span

[Bas S <schalbroeck@xxxxxxxxx>]

I ran into an issue with the i915 driver not being able to drive a display with this specific modeline:

[drm]] Modeline "1920x720": 60 120980 1920 1932 1936 1948 720 723 733 1035 0x48 0x9
[drm:drm_mode_prune_invalid [drm]] Not using 1920x720 mode: H_ILLEGAL

After some investigation I found that intel_mode_valid (and in newer kernels, intel_cpu_transcoder_mode_valid) returns MODE_H_ILLEGAL due to (htotal - hdisplay) being lower than 32.
The modeline in question indeed does not satisfy this constraint, as HTOTAL(1948) - HDISPLAY(1920) equals 28.
Changing the driver code to allow for a hblank span of 28 pixels or lower resulted in the driver successfully rendering to the display.
As such I propose this patch to allow for a tighter hblank span.

Nb: I am uncertain if the hblank span of 32 pixels has been chosen deliberately and what the side-effects could be of lowering this value.
Any insights into this or alternative solutions would be very much appreciated! I also considered introducing a kernel module parameter to optionally loosen these mode constraints.

The referenced modeline is present in a line of ultrawide signage displays and has been known to work on other graphics drivers/OSes.

Signed-off-by: Sebastiaan Schalbroeck <schalbroeck@xxxxxxxxx>
---
 drivers/gpu/drm/i915/display/intel_display.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index b10aad1..f6aba1d 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -7745,13 +7745,13 @@ enum drm_mode_status intel_cpu_transcoder_mode_valid(struct drm_i915_private *de
         */
        if (DISPLAY_VER(dev_priv) >= 5) {
                if (mode->hdisplay < 64 ||
-                   mode->htotal - mode->hdisplay < 32)
+                   mode->htotal - mode->hdisplay < 28)
                        return MODE_H_ILLEGAL;

                if (mode->vtotal - mode->vdisplay < 5)
                        return MODE_V_ILLEGAL;
        } else {
-               if (mode->htotal - mode->hdisplay < 32)
+               if (mode->htotal - mode->hdisplay < 28)
                        return MODE_H_ILLEGAL;

                if (mode->vtotal - mode->vdisplay < 3)

--
2.39.2