linux-kernel - Re: [PATCH v3] i2c: lpi2c: cache peripheral clock rate

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Date:   Tue, 02 May 2023 08:50:06 +0200
From:   Alexander Stein <alexander.stein@...tq-group.com>
To:     Wolfram Sang <wsa@...nel.org>,
        Alexander Stein <alexander.stein@...tq-group.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        NXP Linux Team <linux-imx@....com>,
        "A. Sverdlin" <alexander.sverdlin@...mens.com>,
        Dong Aisheng <aisheng.dong@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>, linux-i2c@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] i2c: lpi2c: cache peripheral clock rate

Hello Wolfgang,

Am Sonntag, 30. April 2023, 09:05:55 CEST schrieb Wolfram Sang:
> * PGP Signed by an unknown key
> 
> > > IIRC this is a general problem^w limitation of the clock framework,
> > > clock providers cannot use clocks themselves in certain callback, e.g.
> > > set_rate.
> > 
> > Well, that's essentially impossible when this clock provider is attached
> > via i2c. i2c transfers potentially need to change or prepare clocks.
> 
> So, as I get it, this is not a specific lpi2c problem but affecting any
> I2C controller driver which uses get_rate() to setup a transfer to a
> remote I2C clock provider? And this lockdep warning is a false-positive?

Yes, IMHO this could potentially occur on every I2C controller driver, if a 
clock provider while holding the clk_prepare_lock, e.g. during registration, 
issues an i2c transfer.
I'm not so sure if this is a false-positive, but more like trying to do a 
nested lock.

Best regards
Alexander
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