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

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Message-ID: <4797405.GXAFRqVoOG@steina-w>
Date:   Mon, 24 Apr 2023 09:03:34 +0200
From:   Alexander Stein <alexander.stein@...tq-group.com>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     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

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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