linux-kernel - Re: [PATCH] vmwgfx: use monotonic event timestamps

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Message-ID: <CAK8P3a2EyP+yx9-4Gr4U0wvGkyTR6F6bnqnecdpuD5D7QNxgeg@mail.gmail.com>
Date:   Thu, 14 Dec 2017 10:05:34 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Sinclair Yeh <syeh@...are.com>
Cc:     VMware Graphics <linux-graphics-maintainer@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        David Airlie <airlied@...ux.ie>,
        Deepak Singh Rawat <drawat@...are.com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Ravikant B Sharma <ravikant.s2@...sung.com>,
        Joe Perches <joe@...ches.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vmwgfx: use monotonic event timestamps

On Wed, Dec 13, 2017 at 9:56 PM, Sinclair Yeh <syeh@...are.com> wrote:
> This looks okay to me.  Although we should change eaction->tv_* type
> to 64-bit as well.

I thought about it but it would add significant complication without real gain,
as the  eaction->tv_* pointers point into uapi structures (drm_vmw_event_fence
and drm_event_vblank) that are defined as 32-bit times and cannot be
changed. Changing the temporary pointer to 64 bit would require adding
a temporary variable to point to and then doing the truncation in the
callers. With monotonic times, we know that 32-bit times are sufficient,
they are good for 136 years after boot (68 years when interpreted as
signed).

       Arnd