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Date:   Wed, 20 Dec 2017 15:35:17 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Alexander.Steffen@...ineon.com, hdegoede@...hat.com,
        linux-kernel@...r.kernel.org
Cc:     james@...le.org.uk, azhar.shaikh@...el.com, arnd@...db.de,
        jarkko.sakkinen@...ux.intel.com, peterhuewe@....de, jgg@...pe.ca,
        gregkh@...uxfoundation.org, linux-integrity@...r.kernel.org
Subject: Re: [PATCH 0/4] tpm: fix PS/2 devices not working on Braswell systems
 due CLKRUN enabled

Hello Alexander,

On 12/20/2017 03:07 PM, Alexander.Steffen@...ineon.com wrote:
>> 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.

I do understand that part. What's not clear to me is if those other issues
as you said exist in practice. My understanding from reading the Low Pin
Count Specification is that the CLKRUN# signal is optional and that the
peripherals that support the CLKRUN protocol should still work even when
the CLKRUN_EN is disabled.

With the disadvantage that the system will be less power efficient of
course.

But yes, the reason why I proposed this minimal change as a fix was to
preserve the behavior of the original patch that caused the regression,
while allowing systems that already have the CLKRUN protocol disabled to
also work (which was what caused the issue since the CLKRUN protocol was
enabled unconditionally after a TPM transaction).

> 
>>> 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.
> 

Yes, I didn't say that this couldn't be solved with a synchronization mechanism
to prevent concurrent access to the LPC control registers. What I meant is that
there isn't such a mechanism in place now so the code is racy and that the lack
of a LPC subsystem forces us to have an (ugly) global lock if we want to fix it.

That's a separate issue though and should be addressed in a different patch-set.

Best regards,
-- 
Javier Martinez Canillas
Software Engineer - Desktop Hardware Enablement
Red Hat

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