Re: [Freedreno] [PATCH 2/2] drm/msm: Add MSM_WAIT_IOVA ioctl

[Jordan Crouse <jcrouse@codeaurora.org>]

On Tue, Jan 14, 2020 at 08:52:43AM -0800, Rob Clark wrote:
> On Mon, Jan 13, 2020 at 9:51 AM Jordan Crouse <jcrouse@codeaurora.org> wrote:
> >
> > On Mon, Jan 13, 2020 at 10:36:05AM -0500, Brian Ho wrote:
> > > +
> > > +     vaddr = base_vaddr + args->offset;
> > > +
> > > +     /* Assumes WC mapping */
> > > +     ret = wait_event_interruptible_timeout(
> > > +                     gpu->event, *vaddr >= args->value, remaining_jiffies);
> >
> > I feel like a barrier might be needed before checking *vaddr just in case you
> > get the interrupt and wake up the queue before the write posts from the
> > hardware.
> >
> 
> if the gpu is doing posted (or cached) writes, I don't think there is
> even a CPU side barrier primitive that could wait for that?  I think
> we rely on the GPU not interrupting the CPU until the write is posted

Once the GPU puts the write on the bus then it is up to the whims of the CPU
architecture. If the writes are being done out of order you run a chance of
firing the interrupt and making it all the way to your handler before the writes
catch up.

Since you are scheduling and doing a bunch of things in between you probably
don't need to worry but if you start missing events and you don't know why then
this could be why. A rmb() would give you piece of mind at the cost of being
Yet Another Barrier (TM).

Jordan

> BR,
> -R
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