linux-kernel - Re: [PATCH v1] iopoll: Tweak readx_poll

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Date:   Thu, 13 Jun 2019 09:11:56 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Marc Gonzalez <marc.w.gonzalez@...e.fr>
Cc:     Arnd Bergmann <arnd@...db.de>, Will Deacon <will.deacon@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Bjorn Helgaas <helgaas@...nel.org>
Subject: Re: [PATCH v1] iopoll: Tweak readx_poll_timeout sleep range

Hi,

On Thu, Jun 13, 2019 at 9:04 AM Marc Gonzalez <marc.w.gonzalez@...e.fr> wrote:
>
> On 13/06/2019 14:42, Arnd Bergmann wrote:
>
> > On Thu, Jun 13, 2019 at 2:16 PM Marc Gonzalez wrote:
> >
> >> Chopping max delay in 4 seems excessive. Let's just cut it in half.
> >>
> >> Signed-off-by: Marc Gonzalez <marc.w.gonzalez@...e.fr>
> >> ---
> >> When max_us=100, old_min was 26 us; new_min would be 50 us
> >> Was there a good reason for the 1/4th?
> >> Is new_min=0 a problem? (for max=1)
> >
> > You normally want a large enough range between min and max. I don't
> > see anything wrong with a factor of four.
>
> Hmmm, I expect the typical use-case to be:
> "HW manual states operation X completes in 100 µs.
> Let's call usleep_range(100, foo); before hitting the reg."
>
> And foo needs to be a "reasonable" value: big enough to be able
> to merge several requests, low enough not to wait too long after
> the HW is ready.
>
> In this case, I'd say usleep_range(100, 200); makes sense.
>
> Come to think of it, I'm not sure min=26 (or min=50) makes sense...
> Why wait *less* than what the user specified?

IIRC usleep_range() nearly always tries to sleep for the max.  My
recollection of the design is that you only end up with something less
than the max if the system was going to wake up anyway.  In such a
case it seems like it wouldn't be insane to go and check if the
condition is already true if 25% of the time has passed.  Maybe you'll
get lucky and you can return early.

Are you actually seeing problems with the / 4, or is this patch just a
result of code inspection?

-Doug