RE: [PATCH 0/4] tpm: fix PS/2 devices not working on Braswell systems due CLKRUN enabled

Shaikh, Azhar
2017-12-20T15:08:44Z

>-----Original Message-----
>From: linux-integrity-owner@vger.kernel.org [mailto:linux-integrity-
>owner@vger.kernel.org] On Behalf Of Alexander.Steffen@infineon.com
>Sent: Wednesday, December 20, 2017 6:07 AM
>To: javierm@redhat.com; hdegoede@redhat.com; linux-
>kernel@vger.kernel.org
>Cc: james@ettle.org.uk; Shaikh, Azhar <azhar.shaikh@intel.com>;
>arnd@arndb.de; jarkko.sakkinen@linux.intel.com; peterhuewe@gmx.de;
>jgg@ziepe.ca; gregkh@linuxfoundation.org; linux-integrity@vger.kernel.org
>Subject: RE: [PATCH 0/4] tpm: fix PS/2 devices not working on Braswell
>systems due CLKRUN enabled
>
>> Hi Hans,
>>
>> Thanks a lot for your feedback.
>>
>> On 12/20/2017 12:43 PM, Hans de Goede wrote:
>> > Hi,
>> >
>> > On 20-12-17 12:35, Javier Martinez Canillas wrote:
>> >> Hello,
>> >>
>> >> Commit 5e572cab92f0 ("tpm: Enable CLKRUN protocol for Braswell
>> systems")
>> >> added logic in the TPM TIS driver to disable the Low Pin Count
>> >> CLKRUN signal during TPM transactions.
>> >>
>> >> Unfortunately this breaks other devices that are attached to the
>> >> LPC bus like for example PS/2 mouse and keyboards.
>> >>
>> >> The bug was reported to the Fedora kernel [0] and the kernel bugzilla [1].
>> >> This issue and the propossed solution were discussed in this [2]
>> >> thread, and the reporter (Jame Ettle) confirmed that his system
>> >> works again after the fix in this series.
>> >>
>> >> The patches are based on top or Jarkko Sakkinen's linux-tpmdd [3] tree.
>> >>
>> >> James,
>> >>
>> >> Even when there shouldn't be any functional changes, I included
>> >> some
>> other
>> >> fixes / cleanups in this series so it would be great if you can
>> >> test them again. I can't because I don't have access to a machine affected
>by this.
>> >>
>> >> [0]: https://bugzilla.redhat.com/show_bug.cgi?id=1498987
>> >> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=197287
>> >> [2]: https://patchwork.kernel.org/patch/10119417/
>> >> [3]: git.infradead.org/users/jjs/linux-tpmdd.git
>> >>
>> >> Best regards,
>> >> Javier
>> >>
>> >>
>> >> Javier Martinez Canillas (4):
>> >> tpm: fix access attempt to an already unmapped I/O memory region
>> >> tpm: delete the TPM_TIS_CLK_ENABLE flag
>> >> tpm: follow coding style for variable declaration in
>> >> tpm_tis_core_init()
>> >> tpm: only attempt to disable the LPC CLKRUN if is already
>> >> enabled
>> >>
>> >> drivers/char/tpm/tpm_tis.c | 17 +----------------
>> >> drivers/char/tpm/tpm_tis_core.c | 24 +++++++++++++++++-------
>> >> drivers/char/tpm/tpm_tis_core.h | 1 -
>> >> 3 files changed, 18 insertions(+), 24 deletions(-)
>> >
>> > Note I'm just reading along here, but I'm wondering if both the TPM
>> > and now also some PS/2 controllers need CLK_RUN to be disabled, why
>> > don't we just disable it once permanently and be done with it?
>> >
>>
>> That's the same question I asked to Azhar who authored the patch that
>> added the CLKRUN toggle logic to the driver, but he didn't answer yet:
>>
>> https://www.spinics.net/lists/linux-integrity/msg01107.html
>>
>> My understanding is that by permanently disabling it, then other
>> devices that do support the CLKRUN protocol will continue to work
>> correctly since the CLKRUN# signal is optional and only used if the host LCLK
>is stopped.
>>
>> The disadvantage though will be that the power management feature of
>> stopping the LPC host LCLK clock when entering into low-power states
>> will not be used.
>>
>> > It seems that on machines with a PS/2 controller connected to the
>> > LPC bus the BIOS is already doing this, so I've a feeling that it
>> > not being done on devices with a TPM is a bug in the firmware
>>
>> Absolutely agree, system integratos should make sure that all the
>> devices connected to the LPC either have CLKRUN protocol support and
>> is enabled or disable the CLKRUN protocol permanently.
>
>As far as I understand it, this is exactly the issue here: They know that there
>are devices that do not support the CLKRUN protocol (the TPM in this case),
>but they still need to enable it to prevent other issues. So for the TPM to
>continue to work, CLKRUN needs to be disabled temporarily while the TPM is
>active.
>

Yes that was the reason to have this fix. We needed CLKRUN to be enabled for Braswell SOC . But the TPM in this case SLB9655 does not support CLKRUN (please check this public documentation https://www.infineon.com/dgdl/Infineon-TPM+SLB+9665-DS-v10_15-EN.pdf?fileId=5546d4625185e0e201518b83d9273d87 section 2.3 Power Management). So as Alexander mentioned CLKRUN is disabled while TPM transactions are in progress.

>> > there and we should just disable it everywhere (and probably find a
>> > better place then the TPM driver to do the disabling).
>> >
>>
>> Indeed. Touching a global bus configuration in a driver for a single
>> device isn't safe to say the least. One problem is that we don't have
>> a LPC bus subsystem so that's why these sort of quirks are done at the driver
>level.
>>
>> > Note this is just an observation, I could be completely wrong here,
>> > but I've a feeling that just disabling CLKRUN all together is the
>> > right thing to do and that seems like an easier fix to me.
>> >
>>
>> I think your observation is correct. The only problem is the power
>> management feature being disabled as mentioned. Although as you said
>> it seems that most BIOS do that (as shown by the patch I posted that
>> just avoids toggling the CLKRUN state if it's already disabled).
>>
>> > Specifically what worries me is: what if another driver also takes
>> > the temporarily disable CLK_RUN approach because of similar issues?
>> > Now we've 2 drivers playing with the CLKRUN state and racing with
>> > each others.
>> >
>>
>> Agreed. You don't even need another driver, if a Braswell system has 2
>> TPMs then you have a race condition and one driver could enable the
>> CLKRUN state while the other driver thinks that's already disabled,
>> and TPM transactions won't work in that case.
>
>Even though it is an unusual configuration to have multiple TPMs in a system,
>this is something that we could fix in the driver by putting locks around the
>state changes, right? And if other drivers also want to change the CLKRUN
>state, at the very least they'd need to use the same (global) lock.
>
>> So yeah, it's not a great situation.
>>
>> > Regards,
>> >
>> > Hans
>> >
>>
>> Best regards,
>> --
>> Javier Martinez Canillas
>> Software Engineer - Desktop Hardware Enablement Red Hat