Re: [Xen-devel] [PATCH] vTPM: Fix Atmel timeout bug.

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 11/10/2014 07:01 AM, Ian Campbell wrote:
> On Thu, 2014-11-06 at 17:01 -0500, Daniel De Graaf wrote:
> > On 11/04/2014 05:15 AM, Ian Campbell wrote:
> > > On Thu, 2014-10-30 at 15:48 +0200, Emil Condrea wrote:
> > > > Of course we can use max, but I thought that it might be useful to
> > > > have a prink to inform the user that the timeout was adjusted.
> > > > In init_tpm_tis the default timeouts are set using:
> > > > /* Set default timeouts */ tpm->timeout_a =
> > > > MILLISECS(TIS_SHORT_TIMEOUT);//750*1000000UL tpm->timeout_b =
> > > > MILLISECS(TIS_LONG_TIMEOUT);//2000*1000000UL tpm->timeout_c =
> > > > MILLISECS(TIS_SHORT_TIMEOUT); tpm->timeout_d =
> > > > MILLISECS(TIS_SHORT_TIMEOUT);
> > > >
> > > >
> > > >
> > > > But in kernel fix they are set as 750*1000 instead of 750*1000000UL :
> > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/char/tpm/tpm_tis.c#n381
> > > > So if we want to integrate kernel changes I think we should use
> > > > MICROSECS(TIS_SHORT_TIMEOUT) which is 750000
> > > > Also in kernel the default timeouts are initialized using
> > > > msecs_to_jiffies which is different from MILLISECS
> > > > macro.: 
> > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/char/tpm/tpm_tis.c#n548
> > > > Is there a certain reason for not using msecs_to_jiffies ?
> > > jiffies are a Linux specific concept which mini-os doesn't share.
> > >
> > > Daniel, do you have any opinion on this patch?
> > >
> > > It seems like the Linux fix is made only for the specifically broken
> > > platform. That seems to make sense to me since presumably other systems
> > > report short timeouts which they can indeed cope with. It's only Atmel
> > > which brokenly reports something it cannot handle.
> > >
> > > Ian.
> > I agree that an adjustment is needed when values are too short.  Adjusting
> > in all cases is not quite as nice as only fixing the broken TPMs, but it
> > is a lot simpler.  It also doesn't seem harmful to have the timeouts be
> > too large in the driver: a properly functioning TPM will not time out its
> > requests in any case, so the user won't notice normally, and the default
> > short timeout is 0.75 seconds - very few people will complain if they have
> > to wait that long to get a timeout instead of what their TPM actually uses.
> Can we take that as an ack?
Yes.  I was going to check to see if the printks would cause people to see
problems where the timeouts were already reasonable, but since the mini-os
driver is already quite verbose, this should not be a problem.  It might
be nice to output the old/new value of the timeout, but that's mostly for
the curious rather than actually necessary.

> Also needs the ok from Konrad as release manager. AIUI this is a bugfix
> for a particular piece of h/w and as Daniel explains above the downside
> is that sometimes someone might need to wait 0.75s for a timeout instead
> of something shorter.
--
Daniel De Graaf
National Security Agency

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