Re: [PATCH v6 01/10] i3c: Add core I3C infrastructure

[Boris Brezillon]

On Thu, 6 Sep 2018 14:59:46 +0200
Arnd Bergmann <arnd@xxxxxxxx> wrote:

> On Thu, Sep 6, 2018 at 2:43 PM Przemyslaw Gaj <pgaj@xxxxxxxxxxx> wrote:
> >
> > Hi Boris, Vitor,
> >
> > This repository does not contain full kernel sources, but it should be enough to discuss mastership request feature.
> > https://github.com/przemekgaj/i3c-linux/commit/d54fe68a9d3e573c0c454a2c6f1afafc20142ec5
> >
> > Please keep in mind that this is initial version, but mastership works correctly. I added one property to DT to reflect relationship between masters.
> > It is possible to request mastership on demand (using sysfs. Useful in case when Linux machine is equipped only with secondary master controller) or automatically change operation mode when device driver wants to read/write something from/to device.
> >
> > I'm sure I will have to rework something because this was implemented on sources from PATCH v4. I saw that Boris released v7 yesterday :)  
> 
> 
> Can you explain the reason for having a user space interface and DT property?
> I thought we had concluded earlier that we wouldn't need that, but it's possible
> that I missed something in the discussion since then.

I don't think the sysfs knob is needed, this being said, after thinking
a bit more about mastership handover and the secondary master case, I
think we have something important to solve.

When a master is not in control of the bus, it gets informed of devices
present on the bus by monitoring DAA or DEFSLVS broadcast events. That
means the secondary master should populate the bus with I3C/I2C devices
on such events, but that's not enough, because DEFSLVS/DAA do not
provide all device info. Some of them (like read/write/ibi limitations)
require extra CCC commands, and, to send those CCC commands, the
secondary master must claim the bus. We could add a case where we
declare devices as partially discovered until the master acquires
ownership of the bus, but that means part of the data returned by
i3c_device_get_info() will be inaccurate, which might have an impact on
some i3c driver ->probe() functions.

We could also say that partially discovered devices should not be
registered to the device model, but we then hit the problem of "who can
force the secondary master to claim the bus if there's no users?".