Re: tpm: read burstcount from TPM_STS in one 32-bit transaction

[Jarkko Sakkinen <jarkko.sakkinen-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Tue, Jul 25, 2017 at 08:17:58PM +0200, Michal Suchánek wrote:
> On Tue, 25 Jul 2017 10:36:11 -0700
> James Bottomley <jejb-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org> wrote:
> 
> > On Tue, 2017-07-25 at 15:04 +0200, Michal Suchánek wrote:
> > > Hello,
> > > 
> > > in commit 9754d45e9970 ("tpm: read burstcount from TPM_STS in one
> > > 32-bit transaction") you change reading of two 8-bit values to one
> > > 32bit read. This is obviously wrong wrt endianess unless the
> > > underlying tpm_tis_read32 does endian conversion.   
> > 
> > Some of the bus read primitives do do endianness conversions.  The
> > problem is with the SPI attachment, which has unclear endianness.  A
> > standard PCI bus attachment uses ioread32() which automatically
> > transforms from a little endian bus to the cpu endianness, however SPI
> > is forced to transfer the bytes one at a time over the serial bus and
> > then transform.  The assumption seems to be that the TIS TPM is
> > replying in little endian format when SPI connected.
> > 
> 
> Yes, that makes sense.
> 
> Thanks for clarification.
> 
> Michal

Thank you for reporting this and thanks James for explaining this.

I do not have access to PPC hardware with SPI-TPM.

/Jarkko

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