Re: [PATCH 1/4] nvmem: rockchip-otp: Serialize reads

From: Miquel Raynal <miquel.raynal@bootlin.com>
To: Alexey Charkov <alchark@flipper.net>
Cc: Srinivas Kandagatla <srini@kernel.org>,
	 Rob Herring <robh@kernel.org>,
	Krzysztof Kozlowski <krzk+dt@kernel.org>,
	 Conor Dooley <conor+dt@kernel.org>,
	 Heiko Stuebner <heiko@sntech.de>,
	 Michael Walle <michael@walle.cc>,
	 Finley Xiao <finley.xiao@rock-chips.com>,
	 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	 devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	 linux-rockchip@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,  stable@vger.kernel.org
Subject: Re: [PATCH 1/4] nvmem: rockchip-otp: Serialize reads
Date: Wed, 02 Sep 2026 12:17:24 +0200	[thread overview]
Message-ID: <87y0dk6jpn.fsf@bootlin.com> (raw)
In-Reply-To: <CAKTNdwH5S3MxgR=5s2LXPquCFRXE-61LTTsZoHP9+YmXo_P4Ag@mail.gmail.com> (Alexey Charkov's message of "Wed, 2 Sep 2026 13:24:00 +0400")

On 02/09/2026 at 13:24:00 +04, Alexey Charkov <alchark@flipper.net> wrote:

> On Wed, Sep 2, 2026 at 1:10 PM Miquel Raynal <miquel.raynal@bootlin.com> wrote:
>>
>> On 01/09/2026 at 19:33:11 +04, Alexey Charkov <alchark@flipper.net> wrote:
>>
>> > The OTP controller is driven through a single set of registers holding a
>> > state machine which has to be stepped through for every word read, yet
>> > nothing keeps two readers out of each other's way. Concurrent reads
>> > interleave, and the outcome is either a reader bailing out:
>> >
>> >   rockchip-otp 2a580000.otp: timeout during read setup
>> >
>> > or, worse, one of them silently taking delivery of the other's data.
>> >
>> > Reading two cells in parallel from userspace on RK3576 reproduces both
>> > within 150 iterations - 53 read errors and 9 corrupted results, the latter
>> > either losing their first word or, in one case, ending in the two bytes
>> > which belong to the other reader's cell - whereas the same reads issued
>> > sequentially never fail. Concurrency is not hypothetical here, as six
>> > thermal sensors source their trim values from the OTP and reach the driver
>> > straight from asynchronous driver probing.
>> >
>> > Guard the read path with a mutex. Reads are the only way into the hardware,
>> > as the driver registers no write callback, and they always run in process
>> > context, so a plain mutex spanning the whole clock-enable, read,
>> > clock-disable sequence is enough.
>> >
>> > Fixes: 755864feb729 ("nvmem: add Rockchip OTP driver")
>> > Cc: stable@vger.kernel.org
>> > Signed-off-by: Alexey Charkov <alchark@flipper.net>
>>
>> Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
>
> Thanks for your review Miquel!
>
> Sashiko complained about mixing a scope-based guard into a function
> with goto-based error handling, so I am replacing the guard(mutex)
> with explicit lock and unlock calls for v2. There won't be any
> semantic change though, so if you don't mind I'd like to carry your
> tag into the v2 version.

Of course.

I haven't seen Sashiko's answer but isn't the goal of guards to just be
released whatever the actual return path?

Thanks,
Miquèl