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

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Message-ID: <0d84fa72-bc96-9b88-cd89-c04327109e0e@ti.com>
Date:   Wed, 4 Dec 2019 20:27:04 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Andrey Smirnov <andrew.smirnov@...il.com>,
        <dri-devel@...ts.freedesktop.org>
CC:     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@...r.kernel.org>, Jyri Sarha <jsarha@...com>
Subject: Re: [PATCH v6 03/15] drm/bridge: tc358767: Simplify polling in
 tc_link_training()

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?

  Tomi

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