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

Open Source and information security mailing list archives

Message-ID: <20200114172319.GA2371@jcrouse1-lnx.qualcomm.com>
Date:   Tue, 14 Jan 2020 10:23:19 -0700
From:   Jordan Crouse <jcrouse@...eaurora.org>
To:     Rob Clark <robdclark@...omium.org>
Cc:     Brian Ho <brian@...ho.com>,
        freedreno <freedreno@...ts.freedesktop.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 Tue, Jan 14, 2020 at 08:52:43AM -0800, Rob Clark wrote:
> 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

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
> _______________________________________________
> Freedreno mailing list
> Freedreno@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/freedreno

-- 
The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives