Re: [PATCH v2 1/2] random: move add_hwgenerator_randomness()'s wait outside function

[Sven van Ashbrook]

On Fri, Sep 16, 2022 at 10:51 AM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
>
> The other thing that occurred to me when reading this patch in context
> of the other one is that this sleep you're removing here is not the
> only sleep in the call chain. Each hwrng driver can also sleep, and
> many do, sometimes for a long time, blocking until there's data
> available, which might happen after minutes in some cases. So maybe
> that's something to think about in context of this patchset -- that
> just moving this to a delayed worker might not actually fix the issue
> you're having with sleeps.
>

This is an excellent point. A look at tpm2_calc_ordinal_duration()
reveals that tpm_transmit() may block for 300s at a time. So when
we are using a WQ_FREEZABLE delayed_work, the PM may have to wait
for up to 300s when draining the wq on suspend. That will introduce
a lot of breakage in suspend/resume.

Dominik: in light of this, please proceed with your patch, without
rebasing it onto mine.

+ tpm maintainers Peter Huewe and Jarkko Sakkinen, a quick recap of
the problem:

- on ChromeOS we are seeing intermittent suspend/resume errors+warnings
  related to activity of the core's hwrng_fillfn. this kthread keeps
  runningduring suspend/resume. if this happens to kick off an bus (i2c)
  transaction while the bus driver is in suspend, this triggers
  a "Transfer while suspended" warning from the i2c core, followed by
  an error return:

i2c_designware i2c_designware.1: Transfer while suspended
tpm tpm0: i2c transfer failed (attempt 1/3): -108
[ snip 10s of transfer failed attempts]

- in 2019, Stephen Boyd made an attempt at fixing this by making the
  hwrng_fillfn kthread freezable. But a freezable thread requires
  different API calls for scheduling, waiting, and timeout. This
  generated regressions, so the solution had to be reverted.

https://patchwork.kernel.org/project/linux-crypto/patch/20190805233241.220521-1-swboyd@xxxxxxxxxxxx/

- the current patch attempts to halt hwrng_fillfn during suspend by
  converting it to a self-rearming delayed_work. The PM drains all
  work before going into suspend. But, the potential minute-long
  blocking delays in tpm make this solution infeasible.

Peter and Jarkko, can you think of a possible way forward to eliminate
the warnings+errors?

-Sven