Re: [PATCH] vTPM: Fix Atmel timeout bug.

[Daniel De Graaf <dgdegra@tycho.nsa.gov>]

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.

-- 
Daniel De Graaf
National Security Agency