Re: [PATCH v2] Documentation: tpm_tis

[Stefan Berger]

On 3/21/24 11:51, Jarkko Sakkinen wrote:
> On Wed Mar 20, 2024 at 6:15 PM EET, Stefan Berger wrote:
> > On 3/20/24 04:56, Jarkko Sakkinen wrote:
> > > Based recent discussions on LKML, provide preliminary bits of tpm_tis_core
> > > dependent drivers. Includes only bare essentials but can be extended later
> > > on case by case. This way some people may even want to read it later on.
> > >
> > > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > > CC: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Lino Sanfilippo <l.sanfilippo@xxxxxxxxxx>
> > > Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> > > Cc: Peter Huewe <peterhuewe@xxxxxx>
> > > Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> > > Cc: Alexander Steffen <Alexander.Steffen@xxxxxxxxxxxx>
> > > Cc: keyrings@xxxxxxxxxxxxxxx
> > > Cc: linux-doc@xxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Cc: linux-integrity@xxxxxxxxxxxxxxx
> > > Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> > > Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > > ---
> > > v2:
> > > - Fixed errors reported by Randy:
> > >     https://lore.kernel.org/all/aed28265-d677-491a-a045-24b351854b24@xxxxxxxxxxxxx/
> > > - Improved the text a bit to have a better presentation.
> > > ---
> > >    Documentation/security/tpm/index.rst   |  1 +
> > >    Documentation/security/tpm/tpm_tis.rst | 30 ++++++++++++++++++++++++++
> > >    2 files changed, 31 insertions(+)
> > >    create mode 100644 Documentation/security/tpm/tpm_tis.rst
> > >
> > > diff --git a/Documentation/security/tpm/index.rst b/Documentation/security/tpm/index.rst
> > > index fc40e9f23c85..f27a17f60a96 100644
> > > --- a/Documentation/security/tpm/index.rst
> > > +++ b/Documentation/security/tpm/index.rst
> > > @@ -5,6 +5,7 @@ Trusted Platform Module documentation
> > >    .. toctree::
> > >    
> > >       tpm_event_log
> > > +   tpm_tis
> > >       tpm_vtpm_proxy
> > >       xen-tpmfront
> > >       tpm_ftpm_tee
> > > diff --git a/Documentation/security/tpm/tpm_tis.rst b/Documentation/security/tpm/tpm_tis.rst
> > > new file mode 100644
> > > index 000000000000..b331813b3c45
> > > --- /dev/null
> > > +++ b/Documentation/security/tpm/tpm_tis.rst
> > > @@ -0,0 +1,30 @@
> > > +.. SPDX-License-Identifier: GPL-2.0
> > > +
> > > +=========================
> > > +TPM FIFO interface Driver
> > > +=========================
> > > +
> > > +FIFO (First-In-First-Out) is the name of the hardware interface used by the
> >
> > FIFO is the type. I am surprised you call it a 'name'. I would say TIS
> > is the 'name'.
>
> It's what the official specification calls it [1].
>
>
> > > +tpm_tis_core dependent drivers. The prefix "tis" comes from the TPM Interface
> >
> > tis is a tla -- a three letter *acronym*. You aren't using it as a 'prefix'.
>
> I don't know what "tla" means.
>
> > > +Specification, which is the hardware interface specification for TPM 1.x chips.
> >
> > It's also available for TPM2.
>   
> Yes, but TIS is the name used by the legacy specification.


The point is that TIS is not just a TPM 1.x interface but also used for 
TPM 2.
> > > +
> > > +Communication is based on a 5 KiB buffer shared by the TPM chip through a
> >
> > I thought it was typically 4 KiB.
>
> You are basing this on table 9 in [1]?

Yes. See below.

> > > +hardware bus or memory map, depending on the physical wiring. The buffer is
> > > +further split into five equal-size buffers, which provide equivalent sets of

If you are referring to the MMIO region between 0xfed4 0000 and 0xfed4 
4fff as a buffer then you are talking about a **20kb** MMIO region 
(0x5000) that is **split** into equal-sized MMIO regions, each having 
4kb (0x1000). Yes, that's the 4kb then but there that one is no 5kb 
'further split into five equal-sized buffers' of presumably 1kb each. 
Each locality has a 0x1000 sized MMIO region.