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Date:   Tue, 13 Apr 2021 22:17:40 +0200
From:   Wolfram Sang <wsa@the-dreams.de>
To:     Qii Wang <qii.wang@mediatek.com>
Cc:     matthias.bgg@gmail.com, linux-i2c@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mediatek@lists.infradead.org, srv_heupstream@mediatek.com,
        leilk.liu@mediatek.com
Subject: Re: [RESEND] i2c: mediatek: Get device clock-stretch time via dts
Message-ID: <20210413201740.GC2751@kunai>
Mail-Followup-To: Wolfram Sang <wsa@the-dreams.de>,
        Qii Wang <qii.wang@mediatek.com>, matthias.bgg@gmail.com,
        linux-i2c@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org,
        srv_heupstream@mediatek.com, leilk.liu@mediatek.com
References: <1615622664-15032-1-git-send-email-qii.wang@mediatek.com>
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On Mon, Apr 12, 2021 at 08:03:14PM +0800, Qii Wang wrote:
> On Wed, 2021-04-07 at 20:19 +0200, Wolfram Sang wrote:
> > > Due to clock stretch, our HW IP cannot meet the ac-timing
> > > spec(tSU;STA,tSU;STO).=20
> > > 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.
> >=20
> > What about using this existing binding?
> >=20
> > - i2c-scl-internal-delay-ns
> >         Number of nanoseconds the IP core additionally needs to setup S=
CL.
> >=20
>=20
> I can't see the relationship between "i2c-scl-falling-time-ns" and clock
> stretching, is there a parameter related to clock stretching?

( you wrote "i2c-scl-falling-time-ns" above, didn't you mean
"i2c-scl-internal-delay-ns" instead? )

Not yet, and I wonder if there can be one. In I2C (not SMBus), devices
are allowed to stretch the clock as long as they want, so what should be
specified here?

I suggesteed "internal-delay" because AFAIU your hardware needs this
delay to be able to cope with clock stretching.

> If you think both of them will affect the ac-timing of SCL, at this
> point, "i2c-scl-falling-time-ns" maybe a good choice.

Do you mean "i2c-scl-falling-time-ns" or "i2c-scl-internal-delay-ns"?


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