[Intel-gfx] TIMESTAMP register

[Lawrynowicz, Jacek]

But how would I synchronize GPU and CPU clocks without access to the TIMESTAMP 
register?
And are you sure that over time both clocks won't desynchronize?

Do you all agree that this functionality should not be implemented in the 
kernel?

Regards,
Jacek

-----Original Message-----
From: Chris Wilson [mailto:chris at chris-wilson.co.uk]
Sent: Wednesday, April 18, 2012 9:36 AM
To: Lawrynowicz, Jacek; Ben Widawsky; Daniel Vetter
Cc: intel-gfx at lists.freedesktop.org
Subject: RE: [Intel-gfx] TIMESTAMP register

On Wed, 18 Apr 2012 06:32:04 +0000, "Lawrynowicz, Jacek" 
<jacek.lawrynowicz at intel.com> wrote:
> I think that ioctl to get GPU timestamp and its resolution would be
> the best approach.
> This ioctl won't be called very often so there's no need to sacrifice
> simplicity for performance in this case.
> Using clock_gettime() sounds like trouble to me.
> How would you keep GPU and CPU time synchronized?
The bspec explicitly says that the GPU timestamp is adjusted for changes in 
render frequency so that it is consistent. Similarly CLOCK_MONOTONIC.

> Is forcewakeup necessary in case of a register which is not affected
> by gfx reset?

They are in the GT powerwell, so yes forcewake will be necessary for the read 
not to return garbage.

The rule of thumb is to keep interfaces out of the kernel unless there is a 
compelling reason otherwise. I think there is a good reason for a 
ioctl(QUERY_COUNTER), but I can also see that you can implement 
proof-of-principle code for this ARB timer entirely in userspace.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre
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