linux-kernel - Re: [RESEND] i2c: mediatek: Get device clock-stretch time via dts

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Date:   Wed, 7 Apr 2021 20:15:06 +0800
From:   Qii Wang <qii.wang@...iatek.com>
To:     Wolfram Sang <wsa@...-dreams.de>
CC:     <matthias.bgg@...il.com>, <linux-i2c@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>,
        <srv_heupstream@...iatek.com>, <leilk.liu@...iatek.com>
Subject: Re: [RESEND] i2c: mediatek: Get device clock-stretch time via dts

On Tue, 2021-04-06 at 21:48 +0200, Wolfram Sang wrote:
> On Sat, Mar 13, 2021 at 04:04:24PM +0800, qii.wang@...iatek.com wrote:
> > From: Qii Wang <qii.wang@...iatek.com>
> > 
> > tSU,STA/tHD,STA/tSU,STOP maybe out of spec due to device
> > clock-stretching or circuit loss, we could get device
> > clock-stretch time from dts to adjust these parameters
> > to meet the spec via EXT_CONF register.
> > 
> > Signed-off-by: Qii Wang <qii.wang@...iatek.com>
> 
> I tried to understand from the code what the new binding expresses, but
> I don't fully understand it. Is it the maximum clock stretch time?
> Because I cannot recall a device which always uses the same delay for
> clock stretching.
> 

Due to clock stretch, our HW IP cannot meet the ac-timing
spec(tSU;STA,tSU;STO). 
There isn't a same delay for clock stretching, so we need pass a
parameter which can be found through measurement to meet most
conditions.