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Subject: Re: [PATCH v3] hwrng: core: Freeze khwrng thread during suspend
From:   Stephen Boyd <swboyd@chromium.org>
Cc:     linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        Andrey Pronin <apronin@chromium.org>,
        Duncan Laurie <dlaurie@chromium.org>,
        Jason Gunthorpe <jgg@ziepe.ca>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Guenter Roeck <groeck@chromium.org>,
        Alexander Steffen <Alexander.Steffen@infineon.com>
To:     Herbert Xu <herbert@gondor.apana.org.au>
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Date:   Tue, 06 Aug 2019 08:01:00 -0700
Sender: linux-crypto-owner@vger.kernel.org
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Quoting Stephen Boyd (2019-08-05 16:32:41)
> The hwrng_fill() function can run while devices are suspending and
> resuming. If the hwrng is behind a bus such as i2c or SPI and that bus
> is suspended, the hwrng may hang the bus while attempting to add some
> randomness. It's been observed on ChromeOS devices with suspend-to-idle
> (s2idle) and an i2c based hwrng that this kthread may run and ask the
> hwrng device for randomness before the i2c bus has been resumed.
>=20
> Let's make this kthread freezable so that we don't try to touch the
> hwrng during suspend/resume. This ensures that we can't cause the hwrng
> backing driver to get into a bad state because the device is guaranteed
> to be resumed before the hwrng kthread is thawed.
>=20
> Cc: Andrey Pronin <apronin@chromium.org>
> Cc: Duncan Laurie <dlaurie@chromium.org>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Guenter Roeck <groeck@chromium.org>
> Cc: Alexander Steffen <Alexander.Steffen@infineon.com>
> Signed-off-by: Stephen Boyd <swboyd@chromium.org>
> ---

Sorry, forgot to add

Fixes: be4000bc4644 ("hwrng: create filler thread")