Re: [PATCH v4 01/10] i3c: Add core I3C infrastructure

[Peter Rosin <peda@axentia.se>]

On July 11, 2018 7:12:12 PM GMT+02:00, Boris Brezillon <boris.brezillon@bootlin.com> wrote:
>On Wed, 11 Jul 2018 17:39:56 +0200
>Arnd Bergmann <arnd@arndb.de> wrote:
>
>> On Wed, Jul 11, 2018 at 4:41 PM, Boris Brezillon
>> <boris.brezillon@bootlin.com> wrote:
>> > On Wed, 11 Jul 2018 16:01:56 +0200 Arnd Bergmann <arnd@arndb.de>
>wrote:  
>> >> > - the bus element is a separate object and is not implicitly
>described
>> >> >   by the master (as done in I2C). The reason is that I want to
>be able
>> >> >   to handle multiple master connected to the same bus and
>visible to
>> >> >   Linux.
>> >> >   In this situation, we should only have one instance of the
>device and
>> >> >   not one per master, and sharing the bus object would be part
>of the
>> >> >   solution to gracefully handle this case.
>> >> >   I'm not sure we will ever need to deal with multiple masters
>> >> >   controlling the same bus and exposed under Linux, but
>separating the
>> >> >   bus and master concept is pretty easy, hence the decision to
>do it
>> >> >   like that.
>> >> >   The other benefit of separating the bus and master concepts is
>that
>> >> >   master devices appear under the bus directory in sysfs.  
>> >>
>> >> I'm not following here at all, sorry for missing prior discussion
>if this
>> >> was already explained. What is the "multiple master" case? Do you
>> >> mean multiple devices that are controlled by Linux and that each
>talk
>> >> to other devices on the same bus, multiple operating systems that
>> >> have talk to are able to own the bus with the kernel being one of
>> >> them, a controller that controls multiple independent buses,
>> >> or something else?  
>> >
>> > I mean several masters connected to the same bus and all exposed to
>the
>> > same Linux instance. In this case, the question is, should we have
>X
>> > I3C buses exposed (X being the number of masters) or should we only
>> > have one?
>> >
>> > Having a bus represented as a separate object allows us to switch
>to
>> > the "one bus : X masters" representation if we need too.  
>> ...
>> >>
>> >> This feels a bit odd: so you have bus_type that can contain
>devices
>> >> of three (?) different device types: i3c_device_type,
>i3c_master_type
>> >> and i3c_busdev_type.
>> >>
>> >> Generally speaking, we don't have a lot of subsystems that even
>> >> use device_types. I assume that the i3c_device_type for a
>> >> device that corresponds to an endpoint on the bus, but I'm
>> >> still confused about the other two, and why they are part of
>> >> the same bus_type.  
>> >
>> > i3c_busdev is just a virtual device representing the bus itself.
>> > i3c_master is representing the I3C master driving the bus. The
>reason
>> > for having a different type here is to avoid attaching this device
>to a
>> > driver but still being able to see the master controller as a
>device on
>> > the bus. And finally, i3c_device are all remote devices that can be
>> > accessed through a given i3c_master.
>> >
>> > This all comes from the design choice I made to represent the bus
>as a
>> > separate object in order to be able to share it between different
>> > master controllers exposed through the same Linux instance. Since
>> > master controllers are also remote devices for other controllers,
>we
>> > need to represent them.  
>> 
>> Ok, so I think this is the most important question to resolve: do we
>> actually need to control multiple masters on a single bus from one OS
>> or not?
>> 
>> The problem that I see is that it breaks the tree abstraction that
>> we use in the dtb interface, in the driver model and in sysfs.
>> If we need to deal with a hardware bus structure like
>> 
>>               cpu
>>              /   \
>>             /     \
>>        platdev   platdev
>>            |        |
>>      i3c-master   i3c-master
>>             \      /
>>              \    /
>>             i3c-bus
>>              /    \
>>          device   device
>> 
>> then that abstraction no longer holds. Clearly you could build
>> a system like that, and if we have to support it, the i3c
>infrastructure
>> should be prepared for it, since we wouldn't be able to retrofit
>> it later.
>
>Exactly. For the DT representation I thought we could have the primary
>master hold the device nodes, and then have secondary masters reference
>the main master with a phandle (i3c-bus = <&main_i3c_master>;). For the
>sysfs representation, it would be the same. Only one of the master
>would create the i3c_bus object and the other masters would just
>reference it.
>
>> 
>> What would be the point of building such a system though?
>
>This, I don't know. But as you said, if we go for a "one bus per
>master" representation, going back will be difficult.
>
>> Is this for performance, failover, or something else?
>
>No, I don't think so, especially since the mastership handover
>operation is not free. So keeping the same master in control is
>probably better in term of perfs.
>
>One case I can think of is when the primary master does not have enough
>resources to address all devices on the bus, and let the secondary
>master handle all transactions targeting those devices.
>
>> IOW, what feature would we lose if we were to declare that
>> setup above invalid (and ensure you cannot represent it in DT)?
>
>That's exactly the sort of discussion I wanted to trigger. Maybe we
>shouldn't care and expose this use case as if it was X different I3C
>buses (with all devices present on the bus being exposed X times to the
>system).

For I2C, this multiple masters for one bus case was retrofitted in the i2c-demux-pinctrl driver. It's a huge kludge with a number of limitations. I don't know it the circumstances for this I2C driver applies for I3C, but with that in mind I'd definitely go with the proposed extra bus object.

Cheers,
Peter