Re: [PATCH v2 0/2] char: tpm: add new driver for tpm i2c ptp

[Oshri Alkobi]

On Thu, Jul 18, 2019 at 8:10 PM Alexander Steffen
<Alexander.Steffen@xxxxxxxxxxxx> wrote:
>
> On 18.07.2019 14:51, Eyal.Cohen@xxxxxxxxxxx wrote:
> > Hi Jarkko and Alexander,
> >
> > We have made an additional code review on the TPM TIS core driver, it looks quite good and we can connect our new I2C driver to this layer.
>
> Great :) In the meantime, I've done some experiments creating an I2C
> driver based on tpm_tis_core, see
> https://patchwork.kernel.org/patch/11049363/ Please have a look at that
> and provide your feedback (and/or use it as a basis for further
> implementations).
>

Sorry for the late response.

Thanks Alexander, indeed it looks much simpler.
I've checked it with Nuvoton's TPM - basic TPM commands work, I only
had to remove the first msg from the read/write I2C transmitting
(from/to TPM_LOC_SEL), the TPM couldn't handle two register writes in
a sequence.
Actually it is more efficient to set TPM_LOC_SEL only before locality
check/request/relinquish - it is sticky.
I still didn't manage to work with interrupts, will debug it.

We weren't aware to the implementation of Christophe/ST which looks
good and can be complement to yours.
If no one is currently working on that, we can prepare a new patch
that is based on both.
Please let us know.

> > However, there are several differences between the SPI interface and the I2C interface that will require changes to the TIS core.
> > At a minimum we thought of:
> > 1. Handling TPM Localities in I2C is different
>
> It turned out not to be that different in the end, see the code
> mentioned above and my comment here:
> https://patchwork.kernel.org/cover/11049365/
>
> > 2. Handling I2C CRC - relevant only to I2C bus hence not supported today by TIS core
>
> That is completely optional, so there is no need to implement it in the
> beginning. Also, do you expect a huge benefit from that functionality?
> Are bit flips that much more likely on I2C compared to SPI, which has no
> CRC at all, but still works fine?
>

I2C is noisy bus with potentially more devices with larger variety
than SPI. I2C may have more than one master and may have collisions
and/or arbitration.
Still we can start w/o CRC for the first stage and add it later.
BTW, Christophe already did most of the work
(https://patchwork.kernel.org/patch/8628661/)

> > 3. Handling Chip specific issues, since I2C implementation might be slightly different across the various TPM vendors
>
> Right, that seems similar to the cr50 issues
> (https://lkml.org/lkml/2019/7/17/677), so there should probably be a
> similar way to do it.

Got it. We hope things will work for us without having another driver,
but it's an option.

>
> > 4. Modify tpm_tis_send_data and tpm_tis_recv_data to work according the TCG Device Driver Guide (optimization on TPM_STS access and send/recv retry)
>
> Optimizations are always welcome, but I'd expect basic communication to
> work already with the current code (though maybe not as efficiently as
> possible).
>
> > Besides this, during development we might encounter additional differences between SPI and I2C.
> >
> > We currently target to allocate an eng. to work on this on the second half of August with a goal to have the driver ready for the next kernel merge window.
> >
> > Regards,
> > Eyal.