Re: [PATCH 0/4] Driver for talking to PLX PEX8xxx PCIe switch over I2C

[Rajat Jain <rajatxjain@gmail.com>]

Hi Wolfram,

Thanks a lot for your review and comments. I understand and will
update with those APIs and send out a V2.

Thanks & Best Regards,

Rajat Jain

On Wed, Oct 1, 2014 at 11:34 PM, Wolfram Sang <wsa@the-dreams.de> wrote:
>
>> I understand and agree. In fact in the internal version of this driver
>> (that I have not yet sent out for review), we do have APIs added
>> similar to what you mention above. Currently I have APIs that:
>> - Enable / Disable PCIe links.
>> - Change the upstream port.
>> - Enable / Disable Non-transparent mode etc.
>
> Now, that sounds better to me...
>
>> However, I did not send them all out for review because I don't have
>> the hardware to try and test them out on ALL the supported devices
>> (also would require considerable amount of time). I have tested those
>> APIs on PEX8713 only, because for e.g.I only have PEX8713 in a HW that
>> connects to 2 CPUs at the same time.
>
> That is a common problem to not have enough hardware to test. You could
> ask on the PCI mailing list for testers. The solution usually lies in
> showing the code rather than not showing the code.
>
>> the switch. Yes, I agree that we can have another layer of abstraction
>> so that we have:
>>
>> - The Core logic code (that knows "How do we want the switch to behave")
>> - A PEX8xxx driver (that knows "which registers to program")
>> - A PEX8xxx I2C driver ("How to program those registers") - this driver.
>>
>> I do understand that your suggestion is to include and bundle the
>> latter two into this same driver.
>
> It definately should be this way. Nobody else than the PEX8xxx driver
> should be able to send I2C messeges to the device! And this is
> absolutely standard, the logic how to talk to a device knows also how to
> prepare the I2C messages. One reason where it could be factored out is
> if there are multiple ways of transportation possible, like I2C or SPI.
>
>> However since the possibilities
>> (about which APIs to provide) are too much and not enough hardware to
>> test it, would it be acceptable if I include those APIs, but support
>> them for only 1 device (and return error for others)?
>
> Start with what YOU need and show us (all of it). From there, we can
> decide: do we start with one driver and factor out later, do we start
> with a sub-subsystem right away, etc... And there is still the question
> what APIs you provide, how they are implemented and if we really should
> have them in-kernel. I think that question will be more interesting for
> Bjorn because I don't really know much about switches in the PCI world.
>
>> with those APIs, I feel exposing the Read/Write APIs will be useful -
>> because what core logic wants to achieve can be highly device and
>> platform specific.
>
> That could also be solved by fixup-callbacks, but we'd need to see the
> core to tell, really.
>
>> Also, a sysfs interface for this switch is proving
>> to be a very helpful development aid :-) (personal experience :-))
>
> Sure, but such development aids don't need to be upstream. Especially if
> they create ABI such as sysfs-entries.
>
> Thanks and regards,
>
>    Wolfram