Re: [tpmdd-devel] tpm: read burstcount from TPM_STS in one 32-bit transaction

[George Wilson]

On Tue, Jul 25, 2017 at 10:36:11AM -0700, James Bottomley 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.
> 
> We can probably get the PPC people to confirm this, I believe they have
> a SPI attached TPM.

All the current OpenPOWER hardware designs I'm aware of have the TPM on
I2C.  Trusted Computing support in OpenPOWER firmware depends on it
being on I2C.

> 
> James
> 
> 
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