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

From: Alexander Stein <alexander.stein@ew.tq-group.com>
To: Wolfram Sang <wsa@kernel.org>,
	Alexander Stein <alexander.stein@ew.tq-group.com>,
	Marc Kleine-Budde <mkl@pengutronix.de>,
	NXP Linux Team <linux-imx@nxp.com>,
	"A. Sverdlin" <alexander.sverdlin@siemens.com>,
	Dong Aisheng <aisheng.dong@nxp.com>,
	Shawn Guo <shawnguo@kernel.org>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Pengutronix Kernel Team <kernel@pengutronix.de>,
	Fabio Estevam <festevam@gmail.com>,
	linux-i2c@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] i2c: lpi2c: cache peripheral clock rate
Date: Tue, 02 May 2023 08:50:06 +0200	[thread overview]
Message-ID: <2143344.irdbgypaU6@steina-w> (raw)
In-Reply-To: <ZE4TU0rCw9MSwrmB@sai>

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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