linux-kernel - Re: [PATCH] tpm: Make timeout logic simpler and more robust

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Message-ID: <20190312125028.GC9243@linux.intel.com>
Date:   Tue, 12 Mar 2019 14:50:28 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     Calvin Owens <calvinowens@...com>, Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] tpm: Make timeout logic simpler and more robust

On Mon, Mar 11, 2019 at 05:27:43PM -0700, James Bottomley wrote:
> On Mon, 2019-03-11 at 16:54 -0700, Calvin Owens wrote:
> > e're having lots of problems with TPM commands timing out, and we're
> > seeing these problems across lots of different hardware (both v1/v2).
> > 
> > I instrumented the driver to collect latency data, but I wasn't able
> > to find any specific timeout to fix: it seems like many of them are
> > too aggressive. So I tried replacing all the timeout logic with a
> > single universal long timeout, and found that makes our TPMs 100%
> > reliable.
> > 
> > Given that this timeout logic is very complex, problematic, and
> > appears to serve no real purpose, I propose simply deleting all of
> > it.
> 
> "no real purpose" is a bit strong given that all these timeouts are
> standards mandated.  The purpose stated by the standards is that there
> needs to be a way of differentiating the TPM crashed from the TPM is
> taking a very long time to respond.  For a normally functioning TPM it
> looks complex and unnecessary, but for a malfunctioning one it's a
> lifesaver.

Standards should be only followed when they make practical sense and
ignored when not. The range is only up to 2s anyway.

/Jarkko