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Subject: Re: [PATCH v2 1/6] hwrng: core: Freeze khwrng thread during suspend
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Peter Huewe <peterhuewe@gmx.de>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-integrity@vger.kernel.org,
        Andrey Pronin <apronin@chromium.org>,
        Duncan Laurie <dlaurie@chromium.org>,
        Guenter Roeck <groeck@chromium.org>,
        Herbert Xu <herbert@gondor.apana.org.au>
From:   Stephen Boyd <swboyd@chromium.org>
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Date:   Wed, 17 Jul 2019 09:42:32 -0700
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Quoting Jason Gunthorpe (2019-07-17 04:39:56)
> On Tue, Jul 16, 2019 at 03:45:13PM -0700, Stephen Boyd wrote:
> > 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
> You mean the TPM here right?

In my case yes, but in general it isn't the TPM.

>=20
> Should we be more careful in the TPM code to check if the TPM is
> suspended before trying to use it, rather than muck up callers?
>=20

Given that it's not just a TPM issue I don't see how checking in the TPM
is going to fix this problem. It's better to not try to get random bytes
from the hwrng when the device could be suspended.