Re: [PATCH 04/11] tpm_crb: use a single read-as-mem/write-as-mmio mapping

[Joelle van Dyne]

On Fri, Jul 14, 2023 at 4:57 AM Stefan Berger <stefanb@linux.ibm.com> wrote:
>
>
>
> On 7/14/23 06:05, Peter Maydell wrote:
> > On Thu, 13 Jul 2023 at 19:43, Stefan Berger <stefanb@linux.ibm.com> wrote:
> >>
> >>
> >>
> >> On 7/13/23 13:18, Peter Maydell wrote:
> >>> On Thu, 13 Jul 2023 at 18:16, Stefan Berger <stefanb@linux.ibm.com> wrote:
> >>>> I guess the first point would be to decide whether to support an i2c bus 
> >>>> on the virt board and then whether we can use the aspeed bus that we 
> >>>> know that the tpm_tis_i2c device model works with but we don't know how 
> >>>> Windows may react to it.
> >>>>
> >>>> It seems sysbus is already supported there so ... we may have a 'match'?
> >>>
> >>> You can use sysbus devices anywhere -- they're just
> >>
> >> 'anywhere' also includes aarch64 virt board I suppose.
> >
> > Yes. Literally any machine can have memory mapped devices.
> >
> >>> "this is a memory mapped device". The question is whether
> >>> we should, or whether an i2c controller is more like
> >>> what the real world uses (and if so, what i2c controller).
> >>>
> >>
> >>> I don't want to accept changes to the virt board that are
> >>> hard to live with in future, because changing virt in
> >>> non-backward compatible ways is painful.
> >>
> >> Once we have the CRB sysbus device we would keep it around forever and it 
> >> seems to
> >> - not require any changes to the virt board (iiuc) since sysbus is already 
> >> being used
> >> - works already with Windows and probably also Linux
> >
> > "Add a sysbus device to the virt board" is the kind of
> > change I mean -- once you do that it's hard to take it
> > out again, and if we decide in 6 months time that actually
> > i2c would be the better option then we end up with two
> > different ways to do the same thing and trying to
> > deprecate the other one is a pain.
>
>
> At least CRB is a standard interface and from this perspective we are fine. I 
> am not sure what would drive the introduction of the i2c bus in 6 months. I 
> suppose one could then still use sysbus CRB device or the i2c device. The 
> sysbus CRB device should still work then. Anyway, I think we should continue 
> with this series.
>
>     Stefan
>
> >
> > -- PMM

FWIW the Windows 11 tpm.sys driver does not support the I2C interface.
The driver only recognizes ACPI devices and the case for Start Method
= 12 (FIFO Interface over I2C bus) goes to an error handler.