linux-kernel - Re: [Freedreno] [PATCH 2/2] drm/msm: Add MSM_WAIT

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Date:   Tue, 14 Jan 2020 08:52:43 -0800
From:   Rob Clark <robdclark@...omium.org>
To:     Brian Ho <brian@...ho.com>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <dri-devel@...ts.freedesktop.org>, Rob Clark <robdclark@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Kristian Kristensen <hoegsberg@...omium.org>,
        Sean Paul <sean@...rly.run>
Subject: Re: [Freedreno] [PATCH 2/2] drm/msm: Add MSM_WAIT_IOVA ioctl

On Mon, Jan 13, 2020 at 9:51 AM Jordan Crouse <jcrouse@...eaurora.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

BR,
-R