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

[Jordan Crouse]

On Tue, Jan 14, 2020 at 09:30:00AM -0800, Kristian Kristensen wrote:
> On Tue, Jan 14, 2020 at 9:23 AM Jordan Crouse <jcrouse@xxxxxxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxxxxxx> 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).
> 
> Doesn't the fence case do the same thing without a barrier?

We get a free __iormb() and dma_rmb() from every gpu_read() so I think that
is enough to catch everything up when the interrupt handler reads the status
register and in most cases we don't check the fence until after that. I'm not
sure that covers all the possible error cases.  Maybe something to look into?

Also that field is marked as volatile in the struct, but I'm not sure that does
anything useful.

Jordan

> > > BR,
> > > -R
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> >
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