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

[Miquel Raynal]

On 01/09/2026 at 19:33:11 +04, Alexey Charkov <alchark at 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 at vger.kernel.org
> Signed-off-by: Alexey Charkov <alchark at flipper.net>

Reviewed-by: Miquel Raynal <miquel.raynal at bootlin.com>