Re: [PATCH v2 4/4] hw/i2c: add pca954x i2c-mux switch

[Philippe Mathieu-Daudé]

+Paolo/Thomas

On 4/9/21 7:21 PM, Patrick Venture wrote:
> On Fri, Apr 9, 2021 at 9:51 AM Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>>
>> Hi Patrick,
>>
>> On 4/9/21 6:25 PM, Patrick Venture wrote:
>>> The pca954x is an i2c mux, and this adds support for two variants of
>>> this device: the pca9546 and pca9548.
>>>
>>> This device is very common on BMCs to route a different channel to each
>>> PCIe i2c bus downstream from the BMC.
>>>
>>> Signed-off-by: Patrick Venture <venture@google.com>
>>> Reviewed-by: Hao Wu <wuhaotsh@google.com>
>>> Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
>>> ---
>>>  MAINTAINERS                      |   6 +
>>>  hw/i2c/Kconfig                   |   4 +
>>>  hw/i2c/i2c_mux_pca954x.c         | 290 +++++++++++++++++++++++++++++++
>>>  hw/i2c/meson.build               |   1 +
>>>  hw/i2c/trace-events              |   5 +
>>>  include/hw/i2c/i2c_mux_pca954x.h |  19 ++
>>>  6 files changed, 325 insertions(+)
>>>  create mode 100644 hw/i2c/i2c_mux_pca954x.c
>>>  create mode 100644 include/hw/i2c/i2c_mux_pca954x.h
>>
>>> diff --git a/hw/i2c/Kconfig b/hw/i2c/Kconfig
>>> index 09642a6dcb..8d120a25d5 100644
>>> --- a/hw/i2c/Kconfig
>>> +++ b/hw/i2c/Kconfig
>>> @@ -28,3 +28,7 @@ config IMX_I2C
>>>  config MPC_I2C
>>>      bool
>>>      select I2C
>>> +
>>> +config PCA954X
>>> +    bool
>>> +    select I2C
>>
>> Do you have a circular dependency when also using:
>>
>>        depends on I2C
>>
>> ?
> 
> I'm somewhat new to qemu -- I don't know what you mean, since I2C
> doesn't depend on pca954x, I don't imagine there could be a circular
> dependency.

See
https://qemu-project.gitlab.io/qemu/devel/kconfig.html#guidelines-for-writing-kconfig-files

PCA954X is plugged on an I2C bus
-> depends on I2C

PCA954X provides I2C buses
-> select I2C

Your device is a particular case consuming and providing the Kconfig
'I2C' symbol. I expect a circular dependency problem. Easy to test with
your series but I haven't.

I suppose in this case, the "select" takes over on "depends on" so this
is OK.

Now (unrelated to your series) thinking at the graphical Kconfig tree
representation (like this one generated 2 years ago:
https://drive.google.com/open?id=1kvwl7guuAmCh2Y2UqeXynlA2HmjWcRs9),
I'd rather see a circular dep.

Regards,

Phil.