vblank problem (and proposed fix) on crtc > 1

Jesse Barnes jbarnes at virtuousgeek.org
Thu Mar 3 16:51:28 PST 2011

On Thu, 3 Mar 2011 17:34:53 -0600 (CST)
Ilija Hadzic <ihadzic at research.bell-labs.com> wrote:

> The fix/improvement I propose is to extend the request.type field
> in drmVBlank structure with additional 5 bits that I call high_crtc
> (there are lots of unused bits in that field). 5 bits covers for 32
> CRTCs, which seems to be the hard limit imposed by various parts of the 
> Xorg and DDX (e.g. possible_crtcs mask in the display descriptor, 
> and the like). If high_crtc is zero, then DRM (kernel module) 
> looks at the primary/secondary flag and maps them to crtc 0 and 1
> (backwards compatibility with older DDX or DDX for other device 
> that does not use the new high_crtc field). If it's not zero then it 
> means that the higher CRTC number is specified by DDX (i.e. userland 
> is a new DDX) and vblank is waited on the specified CRTC (this is used
> only for crtc > 1, crtc 0 and 1, still use the old style).

Yeah, I think that should work, though another option would be to just
add a new ioctl.  That would make compat checking easy for new code; it
could just call the new ioctl and if that returned -ENOTTY it could
fall back to the old one and throw away the CRTC info or complain if
the count was too high.

But you're right that when we re-wrote the code we fixed it to handle >
2 CRTCs, so it should be mostly ready for that (modulo testing, which
it sounds like you're doing already).


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