Ignore zero length gamma curves

[AlanGriffiths]

After a wake we might see an invalid Gamma curve. Ignore it.

Fixes: #3238

It fixes the problem because ensure_crtc() might allocate a new current_crtc but the drmModeCrtcSetGamma() will fail leaving it unused - but still associated with the output.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d0a68ae) 77.81% compared to head (0a89793) 77.83%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3248      +/-   ##
==========================================
+ Coverage   77.81%   77.83%   +0.02%     
==========================================
  Files        1058     1068      +10     
  Lines       67163    67565     +402     
==========================================
+ Hits        52265    52592     +327     
- Misses      14898    14973      +75     

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[RAOF]

This change certainly isn't wrong, but why are we attempting to set a zero-sized gamma curve? What's trying to set this further up the stack?

[AlanGriffiths]

why are we attempting to set a zero-sized gamma curve? What's trying to set this further up the stack?

I wondered that too. I've not been able to identify specific wrong logic. It like we default construct it and never change it, and I don't see anything that guarantees we will change it so...

Will have a further look, but IMO this mitigation is good anyway

[AlanGriffiths]

Will have a further look, but IMO this mitigation is good anyway

The only place gamma gets set is mgg::RealKMSOutput::update_from_hardware_state() and there it is set by this dubious path:

    GammaCurves gamma;

    /* Get information about the current mode */
    if (current_crtc) {
        current_mode_info = current_crtc->mode;

        if (current_crtc->gamma_size > 0)
            gamma = mg::LinearGammaLUTs(current_crtc->gamma_size);
    }
...
    output.gamma = gamma;

The upshot of which is, that if there no current_crtc then it is set to the default constructed GammaCurves gamma. (Even if it had been set previously - which I doubt.)

And, empirically, the usual way to reach this code is with current_crtc false.