Re: [Patch v5.1 03/03]: hwrng: khwrngd derating per device

[Torsten Duwe <duwe@suse.de>]

On Sun, Jun 15, 2014 at 01:11:46AM -0400, Theodore Ts'o wrote:
> OK, I've merged these changes into the random.git tree.
> 
> I had to make a few minor changes.
> 
> 1)  Changes so it would compile on 3.15.  (random_write_wakeup_thresh
> got renamed to random_write_wakeup_bits).  I'm guessing the patch was
> massaged so that it would apply, but it was never compile tested.

I'm keeping and updating 2 versions, one -current (more or less)
and one for 3.12. I probably missed that when making the discussion changes
back and forth, sorry.

> 2)  Fixed a bug in patch #2 so that it would work correctly if the rng
> driver doesn't have an init function (which happens to be the case for
> the tpm-rng driver, which I used for my testing).

The whole thing stems from entropy-challenged s390. 3.12 on s390 compiles
and runs fine. Yields a solid 200 kB/s

TPM RNG is a crook ;-)

> There are also a few minor rough edges that I've noted, but not yet
> fixed.  The main one is that if you've compiled the hw_random's
> rng_core into the kernel, changes to
> /sys/modules/rng_core/parameters/* won't actually cause the hwrngd
> kerenl thread to get started.  You have to set the parameters before
> you load the rng module in order for them to be activated.  And if
> you've compiled the rng module into the kernel, that trick won't work.

With patch 03/03, it is up to the driver author to specify an entropy
quality, which can be overridden at boot time, or when loading
the module, respectively. This should be a constant hardware property.
It would be nice to change it at runtime; but frankly I hope that this
won't be neccessary.

> Fixing this probably means that we need to set up a formal sysfs tree
> under /sys/kernel/hw_random.

Maybe along with more sophisticated steering of how many bits to pick
from which source, if multiple are available.

Thanks,

	Torsten