modetest: document why no blob is created for linear gamma LUT

As found and discussed in [MR 58] a blob is not created in the else arm
because adding the GAMMA_LUT property with a NULL/0 blob_id causes it
to be reset to a default linear / pass-thru gamma table.  The values
in the gamma_lut table might still be consumed in the legacy API path
below though, so it has to be initialized to a linear table.

[MR 58]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/58#note_466972

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>

diff --git a/tests/modetest/modetest.c b/tests/modetest/modetest.c
index 3d24e0d..8bc3d5a 100644 (file)
--- a/tests/modetest/modetest.c
+++ b/tests/modetest/modetest.c
@@ -1158,6 +1158,11 @@ static void set_gamma(struct device *dev, unsigned crtc_id, unsigned fourcc)
                util_smpte_c8_gamma(256, gamma_lut);
                drmModeCreatePropertyBlob(dev->fd, gamma_lut, sizeof(gamma_lut), &blob_id);
        } else {
+               /*
+                * Initialize gamma_lut to a linear table for the legacy API below.
+                * The modern property API resets to a linear/pass-thru table if blob_id
+                * is 0, hence no PropertyBlob is created here.
+                */
                for (i = 0; i < 256; i++) {
                        gamma_lut[i].red =
                        gamma_lut[i].green =