linux-kernel - Re: [PATCH 5/9] drm/bridge: tc358767: Simplify polling in tc_link

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Message-ID: <20190312151515.GA17924@pendragon.ideasonboard.com>
Date:   Tue, 12 Mar 2019 17:15:15 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Andrey Smirnov <andrew.smirnov@...il.com>
Cc:     dri-devel@...ts.freedesktop.org,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Chris Healy <cphealy@...il.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/9] drm/bridge: tc358767: Simplify polling in
 tc_link_training()

Hi Andrey,

On Mon, Mar 11, 2019 at 11:26:20AM -0700, Andrey Smirnov wrote:
> On Mon, Mar 4, 2019 at 4:30 AM Laurent Pinchart wrote:
> > On Tue, Feb 26, 2019 at 11:36:05AM -0800, Andrey Smirnov wrote:
> >> Replace explicit polling in tc_link_training() with equivalent call to
> >> regmap_read_poll_timeout() for simplicity. No functional change
> >> intended (not including slightly altered debug output).
> >>
> >> Signed-off-by: Andrey Smirnov <andrew.smirnov@...il.com>
> >> Cc: Archit Taneja <architt@...eaurora.org>
> >> Cc: Andrzej Hajda <a.hajda@...sung.com>
> >> Cc: Laurent Pinchart <Laurent.pinchart@...asonboard.com>
> >> Cc: Chris Healy <cphealy@...il.com>
> >> Cc: Lucas Stach <l.stach@...gutronix.de>
> >> Cc: dri-devel@...ts.freedesktop.org
> >> Cc: linux-kernel@...r.kernel.org
> >> ---
> >>  drivers/gpu/drm/bridge/tc358767.c | 14 +++++---------
> >>  1 file changed, 5 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/bridge/tc358767.c b/drivers/gpu/drm/bridge/tc358767.c
> >> index 6455e6484722..ea30cec4a0c3 100644
> >> --- a/drivers/gpu/drm/bridge/tc358767.c
> >> +++ b/drivers/gpu/drm/bridge/tc358767.c
> >> @@ -735,7 +735,6 @@ static int tc_link_training(struct tc_data *tc, int pattern)
> >>       const char * const *errors;
> >>       u32 srcctrl = tc_srcctrl(tc) | DP0_SRCCTRL_SCRMBLDIS |
> >>                     DP0_SRCCTRL_AUTOCORRECT;
> >> -     int timeout;
> >>       int retry;
> >>       u32 value;
> >>       int ret;
> >> @@ -765,20 +764,17 @@ static int tc_link_training(struct tc_data *tc, int pattern)
> >>               tc_write(DP0CTL, DP_EN);
> >>
> >>               /* wait */
> >> -             timeout = 1000;
> >> -             do {
> >> -                     tc_read(DP0_LTSTAT, &value);
> >> -                     udelay(1);
> >> -             } while ((!(value & LT_LOOPDONE)) && (--timeout));
> >> -             if (timeout == 0) {
> >> +             ret = regmap_read_poll_timeout(tc->regmap, DP0_LTSTAT, value,
> >> +                                            value & LT_LOOPDONE, 1, 1000);
> >> +             if (ret) {
> >>                       dev_err(tc->dev, "Link training timeout!\n");
> >>               } else {
> >>                       int pattern = (value >> 11) & 0x3;
> >>                       int error = (value >> 8) & 0x7;
> >>
> >>                       dev_dbg(tc->dev,
> >> -                             "Link training phase %d done after %d uS: %s\n",
> >> -                             pattern, 1000 - timeout, errors[error]);
> >> +                             "Link training phase %d done: %s\n",
> >> +                             pattern, errors[error]);
> >
> > It's probably not a big deal in this specific case, but in general it
> > can be useful to know how long the poll took.
> 
> I don't disagree, but bear in mind that the way time is measured in
> original loop assumes that tc_read, an I2C transaction over 100KHz
> bus, takes insignificant amount of time compared to 1 uS delay. I
> think original debug statement does a bit of a false advertising when
> it presents a number of polling loop iterations as if it is time it
> took to establish a link in microseconds.
> 
> > Any hope to enhance regmap_read_poll_timeout() to return either the elapsed time or the
> > remaining timeout instead of 0 on success ?
> 
> I'd rather not go there. That'll take convincing Mark Brown to accept
> that semantics change, then fixing all of the callers across the tree
> via a separate patch series.
> 
> What if instead we just add an extra debug statement before link
> training starts, so that duration of the process can be discerned from
> logging timestamps? This does require user doing a bit of math by
> hand, but it's actually more accurate timing info compared to original
> and it doesn't require any API modification.

That works for me.

-- 
Regards,

Laurent Pinchart