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

[Stefan Berger <stefanb@linux.ibm.com>]

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