linux-kernel - Re: [PATCH 2/2] i2c: rcar: add FastMode+ support for Gen4

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Message-ID: <ZQm36a4IhPL9kN+g@shikoro>
Date:   Tue, 19 Sep 2023 17:02:01 +0200
From:   Wolfram Sang <wsa+renesas@...g-engineering.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     linux-renesas-soc@...r.kernel.org,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Andi Shyti <andi.shyti@...nel.org>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] i2c: rcar: add FastMode+ support for Gen4

> > +               /* FM+ needs lower SMD and no filters */
> > +               priv->smd /= 2;
> > +               cdf = 0;
> 
> Is this documented somewhere in the R-Car Gen4 docs?

Sadly, there are no recommended values for FM+ in the docs. With cdf=6,
I got speeds around 800kHz. After seeing the BSP also uses cdf=0, I got
speeds close to 1MHz. There is a note in Gen3 and later docs for ICCCR2
saying that cdf=0 is recommended for devices with a data hold time less
than 100ns. I don't have the PMIC docs here to verify if that is the
case here.

However, I recalled there are some generic filter bindings for I2C. I
will check if we can make use of them here.

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