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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>
main
Marijn Suijten 2023-09-09 10:26:33 +02:00
parent b709c3010e
commit 64b09cef49
1 changed files with 5 additions and 0 deletions
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5
tests/modetest/modetest.c
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@ -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 =