[PATCH] xf86-video-ati: vblank wait on crtc > 1

Ilija Hadzic ihadzic at research.bell-labs.com
Tue Mar 22 07:53:59 PDT 2011

On Tue, 22 Mar 2011, Michel [ISO-8859-1] Dänzer wrote:

> In the post I referenced above, you said:
>> [...] I'll add a hook to the DDX to check the version and not issue
>> the ioctl at all if it is requested on a higher CRTC. I think that's
>> better than falling back to the old style and issuing the system call
>> on (potentially wrong) CRTC #1 because that can block the application
>> (and I'd rather see it proceed without attempting vblank
>> synchronization, then block).
> Which made sense then and still does now, in contrast to trying to
> preserve ill-defined broken behaviour.

... and then as I started to write code I changed my mind (am I forgiven ? 
;-) ) because I realized that three things would happen:

a) just not issuing the ioctl will cause an application to "firehose" 
the kernel with rendering commands and potentially impact the performance 
of other processes.

b) both behaviors (not issuing the ioctl and thus causing application to 
keep coming back after glxSwap or just blocking the application on
bad CRTC) are broken anyway so introducing a wider variety of 
broken behaviors was probably worse as far as user's experience is 

c) the change the way I made it is safer wrt. introducing new bugs; you 
can't as easily fake out the sequence number and timestamp as you may 

I explained all of the above in my followup posts and although I didn't 
want to repeat them now, I now find myself repeating it anyway ;-)

>>>> +	} else {
>>>> +	    if (cap_value) {
>>>> +		info->high_crtc_works = TRUE;
>>>> +	    } else {
>>>> +		xf86DrvMsg(pScrn->scrnIndex, X_WARNING, "Your kernel does not handle VBLANKs on CRTC>1\n");
>>>> +		info->high_crtc_works = FALSE;
>>> Is there any point in having two different warning messages? I think
>>> 'CRTC > 1' could use spaces.
>> There is a point: one warning tells you that the kernel is old and you
>> have to upgrade. The other warning tells you that the kernel is new enough
>> (it has the GET_CAP ioctl), but for some other reason refused to handle
>> high-crtcs (which at this time doesn't exist, but it should not be the
>> reason to "destroy" the information).
> Fair enough.
>> I bet the change on my desk in my office that if I had the blankspace,
>> someone would have responded with an opposite suggestion ;-).
> That's bold of you. I stand by my request.

Next time I touch this file, I'll "smuggle" the blankspace, but let's say 
that this is a rock bottom of the priority list (just as fixing grammar, 
style and spelling in any message would be  ... and there is plenty).

-- Ilija

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