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Date: Sun, 14 Apr 2024 00:41:29 +0300
Message-Id: <D0JBLACWBFZZ.2ZYOFBJVG5EVU@kernel.org>
Cc: "Alexander Steffen" <Alexander.Steffen@infineon.com>, "Daniel P . Smith"
 <dpsmith@apertussolutions.com>, "James Bottomley"
 <James.Bottomley@hansenpartnership.com>, "Jason Gunthorpe" <jgg@ziepe.ca>,
 "Jonathan Corbet" <corbet@lwn.net>, "Lino Sanfilippo"
 <l.sanfilippo@kunbus.com>, "Mimi Zohar" <zohar@linux.ibm.com>, "Peter
 Huewe" <peterhuewe@gmx.de>, "Randy Dunlap" <rdunlap@infradead.org>,
 <linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] Documentation: tpm_tis
From: "Jarkko Sakkinen" <jarkko@kernel.org>
To: "Bagas Sanjaya" <bagasdotme@gmail.com>,
 <linux-integrity@vger.kernel.org>
References: <20240409190847.10869-1-jarkko@kernel.org>
 <20240409190847.10869-3-jarkko@kernel.org> <ZhfAYVLwoYAPnYbI@archie.me>
In-Reply-To: <ZhfAYVLwoYAPnYbI@archie.me>

On Thu Apr 11, 2024 at 1:50 PM EEST, Bagas Sanjaya wrote:
> On Tue, Apr 09, 2024 at 10:08:47PM +0300, Jarkko Sakkinen wrote:
> > diff --git a/Documentation/security/tpm/tpm_tis.rst b/Documentation/sec=
urity/tpm/tpm_tis.rst
> > new file mode 100644
> > index 000000000000..b448ea3db71d
> > --- /dev/null
> > +++ b/Documentation/security/tpm/tpm_tis.rst
> > @@ -0,0 +1,46 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D
> > +TPM FIFO interface driver
> > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D
> > +
> > +TCG PTP Specification defines two interface types: FIFO and CRB. The f=
ormer is
> > +based on sequenced read and write operations,  and the latter is based=
 on a
> > +buffer containing the full command or response.
> > +
> > +FIFO (First-In-First-Out) interface is used by the tpm_tis_core depend=
ent
> > +drivers. Originally Linux had only a driver called tpm_tis, which cove=
red
> > +memory mapped (aka MMIO) interface but it was later on extended to cov=
er other
> > +physical interfaces supported by the TCG standard.
> > +
> > +For legacy compliance the original MMIO driver is called tpm_tis and t=
he
> Did you mean "For historical reasons above ..."?

That would be better wording.

> > +framework for FIFO drivers is named as tpm_tis_core. The postfix "tis"=
 in
> > +tpm_tis comes from the TPM Interface Specification, which is the hardw=
are
> > +interface specification for TPM 1.x chips.
> > +
> > +Communication is based on a 20 KiB buffer shared by the TPM chip throu=
gh a
> > +hardware bus or memory map, depending on the physical wiring. The buff=
er is
> > +further split into five equal-size 4 KiB buffers, which provide equiva=
lent
> > +sets of registers for communication between the CPU and TPM. These
> > +communication endpoints are called localities in the TCG terminology.
> > +
> > +When the kernel wants to send commands to the TPM chip, it first reser=
ves
> > +locality 0 by setting the requestUse bit in the TPM_ACCESS register. T=
he bit is
> > +cleared by the chip when the access is granted. Once it completes its
> > +communication, the kernel writes the TPM_ACCESS.activeLocality bit. Th=
is
> > +informs the chip that the locality has been relinquished.
> > +
> > +Pending localities are served in order by the chip in descending order=
, one at
> > +a time:
> > +
> > +- Locality 0 has the lowest priority.
> > +- Locality 5 has the highest priority.
> > +
> > +Further information on the purpose and meaning of the localities can b=
e found
> > +in section 3.2 of the TCG PC Client Platform TPM Profile Specification=
.
> > +
> > +References
> > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> > +
> > +TCG PC Client Platform TPM Profile (PTP) Specification
> > +https://trustedcomputinggroup.org/resource/pc-client-platform-tpm-prof=
ile-ptp-specification/
>
> Other than that,
>
> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>


Thanks! I'll apply this with the fix you proposed.

For everyone: this is by no means perfect. The point is to seed
something we can build on top of. So I leave it rather lacking stuff
than try to document every possible bells and whistle. This can be
then improved based on discussions and future patch sets.

BR, Jarkko