Re: [PATCH 1/3] drm: Widen vblank count to 64 bits. Change vblank time precision to ns

From: Michel DÃnzer
Date: Thu Jul 06 2017 - 04:06:11 EST

On 06/07/17 04:45 PM, Michel DÃnzer wrote:
> On 06/07/17 07:10 AM, Keith Packard wrote:
>> This modifies the datatypes used by the vblank code to provide both 64
>> bits of vblank count and to increase the resolution of the vblank
>> timestamp from microseconds to nanoseconds.
>> The driver interfaces have also been changed to return 64-bits of
>> vblank count; fortunately all of the code necessary to widen that value
>> was already included to handle devices returning fewer than 32-bits.
>> This will provide the necessary datatypes for the Vulkan API.
>> Signed-off-by: Keith Packard <keithp@xxxxxxxxxx>
> [...]
>> @@ -1492,9 +1515,11 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
>> switch (vblwait->request.type & _DRM_VBLANK_TYPES_MASK) {
>> - vblwait->request.sequence += seq;
>> + req_seq = seq + vblwait->request.sequence;
>> vblwait->request.type &= ~_DRM_VBLANK_RELATIVE;
> Subtle breakage here: vblwait->request.sequence must still get updated
> for _DRM_VBLANK_RELATIVE, in case we're interrupted by a signal.

BTW, this got me thinking that we should probably treat
_DRM_VBLANK_NEXTONMISS the same way, i.e. clear the flag after updating
vblwait->request.sequence. Otherwise there could theoretically (though
unlikely) be an infinite loop:

ioctl with _DRM_VBLANK_NEXTONMISS, target missed => wait for next vblank
wait interrupted by signal
lather, rinse, repeat

I'd advise against adding a "next on miss" flag for the new ioctl until
there is specific demand for that.

Earthling Michel DÃnzer |
Libre software enthusiast | Mesa and X developer