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

From: Alexander Stein <alexander.stein@ew.tq-group.com>
To: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: 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: Mon, 24 Apr 2023 09:03:34 +0200	[thread overview]
Message-ID: <4797405.GXAFRqVoOG@steina-w> (raw)
In-Reply-To: <20230421-kinfolk-glancing-e185fd9c47b4-mkl@pengutronix.de>

Hi Marc,

Am Freitag, 21. April 2023, 15:59:52 CEST schrieb Marc Kleine-Budde:
> * PGP Signed by an unknown key
> 
> On 21.04.2023 15:48:59, Alexander Stein wrote:
> [...]
> 
> > Now lpi2c_runtime_resume will call into clk_prepare() which also calls
> > clk_prepare_lock() (identical to clk_get_rate).
> 
> 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.

One problem is that during clock registration (__clk_core_init) in the call to 
clk_ops.recalc_rate an i2c transfer is issued (via regmap). This is the 
inverse lock order to a regular transfer runtime resuming the i2c master and 
preparing the clocks.

While looking at this, is_prepared is also affected by this. It's reading a 
register, thus issuing a i2c transfer. But this function can also be called 
from within clk_unprepare_unused_subtree, which holds clk_prepare_lock as 
well.

This might be avoided by using regcache, but I'm not really sure this catches 
every case.

Best regards,
Alexander
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