linux-kernel - Re: [PATCH v6 03/15] drm/bridge: tc358767: Simplify polling in tc_link

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Date:   Thu, 5 Dec 2019 08:06:00 -0800
From:   Andrey Smirnov <andrew.smirnov@...il.com>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     dri-devel@...ts.freedesktop.org,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Andrey Gusakov <andrey.gusakov@...entembedded.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Cory Tusar <cory.tusar@....aero>,
        Chris Healy <cphealy@...il.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jyri Sarha <jsarha@...com>
Subject: Re: [PATCH v6 03/15] drm/bridge: tc358767: Simplify polling in tc_link_training()

On Wed, Dec 4, 2019 at 10:27 AM Tomi Valkeinen <tomi.valkeinen@...com> wrote:
>
> Hi Andrey,
>
> On 19/06/2019 08:27, Andrey Smirnov wrote:
>
> > @@ -748,22 +748,19 @@ static int tc_set_video_mode(struct tc_data *tc,
> >
> >   static int tc_wait_link_training(struct tc_data *tc)
> >   {
> > -     u32 timeout = 1000;
> >       u32 value;
> >       int ret;
> >
> > -     do {
> > -             udelay(1);
> > -             tc_read(DP0_LTSTAT, &value);
> > -     } while ((!(value & LT_LOOPDONE)) && (--timeout));
> > -
> > -     if (timeout == 0) {
> > +     ret = tc_poll_timeout(tc, DP0_LTSTAT, LT_LOOPDONE,
> > +                           LT_LOOPDONE, 1, 1000);
>
> This seems to break DP at least with some monitors for me. I think it's just a timeout problem, as
> increasing the values helps.
>
> Using ktime, I can see that during link training, the first call takes ~2ms, the second ~7ms. I
> think this worked before, as udelay(1) takes much longer than 1 us.
>
> We have 1000us limit in a few other places too, which I don't see causing issues, but might need
> increasing too.
>
> Also, 1us sleep_us may be a bit too small to be sane. If the loops take milliseconds, probably 100us
> or even more would make sense.
>
> This didn't cause any issues with your display?
>

Hmm, not that I know of. Your reasoning makes sense, though. If
increasing the timeout helps, I am all for it. And, yeah, I agree,
this is probably not the only place that could use an increased
timeout.

Thanks,
Andrey Smirnov