linux-kernel - Re: [PATCH] drm/msm: avoid using 'timespec'

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Message-ID: <20180711140024.GV20303@art_vandelay>
Date:   Wed, 11 Jul 2018 10:00:24 -0400
From:   Sean Paul <seanpaul@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Sean Paul <seanpaul@...omium.org>, Rob Clark <robdclark@...il.com>,
        David Airlie <airlied@...ux.ie>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        linux-arm-msm@...r.kernel.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno@...ts.freedesktop.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm: avoid using 'timespec'

On Tue, Jul 10, 2018 at 11:27:00PM +0200, Arnd Bergmann wrote:
> On Tue, Jul 10, 2018 at 10:47 PM, Sean Paul <seanpaul@...omium.org> wrote:
> > On Mon, Jun 18, 2018 at 05:39:42PM +0200, Arnd Bergmann wrote:
> >> The timespec structure and associated interfaces are deprecated and will
> >> be removed in the future because of the y2038 overflow.
> >>
> >> The use of ktime_to_timespec() in timeout_to_jiffies() does not
> >> suffer from that overflow, but is easy to avoid by just converting
> >> the ktime_t into jiffies directly.
> >>
> >> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> >> ---
> >>  drivers/gpu/drm/msm/msm_drv.h | 3 +--
> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
> >> index b2da1fbf81e0..cc8977476a41 100644
> >> --- a/drivers/gpu/drm/msm/msm_drv.h
> >> +++ b/drivers/gpu/drm/msm/msm_drv.h
> >> @@ -353,8 +353,7 @@ static inline unsigned long timeout_to_jiffies(const ktime_t *timeout)
> >>               remaining_jiffies = 0;
> >>       } else {
> >>               ktime_t rem = ktime_sub(*timeout, now);
> >> -             struct timespec ts = ktime_to_timespec(rem);
> >> -             remaining_jiffies = timespec_to_jiffies(&ts);
> >> +             remaining_jiffies = ktime_divns(rem, NSEC_PER_SEC / HZ);
> >
> > Do you need to wrap rem in ktime_to_ns() just to be safe?
> 
> The ktime_t interfaces are still defined to use an opaque type,
> as previously it was a union that could be a seconds/nanoseconds
> pair depending on the architecture. These days, ktime_t is just
> a 64-bit integer, so div_u64() would work just as well as ktime_divns(),
> but this is the documented way to do it.

Hey Arnd,
Ahh, ok, I think I realize my confusion now. If ktime_t was not ns, 
ktime_divns() would do the conversion for us. Since it is ns, the conversion
is a no-op (which is why I didn't see ktime_to_ns() in ktime_divns()).

Thanks for breaking that down for me,

Reviewed-by: Sean Paul <seanpaul@...omium.org>


> 
>       Arnd

-- 
Sean Paul, Software Engineer, Google / Chromium OS