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Date:   Fri, 20 Jul 2018 15:17:51 +0200
From:   Boris Brezillon <boris.brezillon@bootlin.com>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Peter Rosin <peda@axentia.se>, Wolfram Sang <wsa@the-dreams.de>,
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Subject: Re: [PATCH v6 00/10] Add the I3C subsystem
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On Fri, 20 Jul 2018 13:28:10 +0200
Arnd Bergmann <arnd@arndb.de> wrote:

> On Fri, Jul 20, 2018 at 1:13 PM, Peter Rosin <peda@axentia.se> wrote:
> > On 2018-07-20 12:57, Arnd Bergmann wrote:  
> >> * What I understand from reading i2c-demux-pinctrl.c, a slave device
> >>   will only ever be observable from one master at a time, when you
> >>   switch over, all children get removed on one master and added to
> >>   the other one, to be probed again by their respective drivers.
> >>   I can see this as a useful feature on i3c as well, in particular to
> >>   deal with the situation where we have i2c slaves connected to a
> >>   pinmux that can switch them between an i3c master and an
> >>   i2c-only master (possibly a gpio based one). That particular use
> >>   case however doesn't seem to fix well in the current code, which
> >>   is structure around i3c buses.  
> >
> > It's pretty easy to come up with examples where this reprobing is
> > not desirable at all. E.g. if one of the involved I2C devices is
> > a HDMI encoder (I have a TDA19988 here) sitting in the middle of the
> > graphics pipeline. Blink-blink on the screen because some *other*
> > unrelated device needed to be accessed by an alternative master. Not
> > pretty.  
> 
> Agreed, we definitely don't want to reprobe all devices during normal
> operation for i3c master handover.
> 

Re-probing would not happen, no matter the solution we choose. It's
that, in one case, you would have X virtual/linux devices representing
the same physical device and in the other case, you would just have
one, and everytime a transfer is requested by the driver, the core
would pick the appropriate master to do it (most likely the one in
control of the bus at that time)

> What is the least contrived use case that you can think of where we
> would want to use one master to talk to one device on the bus,
> but another master to talk to another device on the same bus?

The only reason I see for this use case is when you have 2 masters,
each of them having different capabilities:

device A needs cap X
device B need cap Y
master N provides cap X but not cap Y
master M provides cap Y but not cap X

In this case you'd want device A to use master M and device B to use
master N.

> I still hope that we can decide that this is not a useful scenario
> at all.

I'm not saying this scenario is about to happen IRL, just describing
one potential reason for having 2 different masters connected to the
same bus and both exposed to Linux.

> 
> If we find a case in which it is needed, we could still deal with it
> like this:
> - enumerate all slaves connected to the bus for each of the
>   two masters

That's what will happen if you don't share the same bus representation.

> - mark each slave as status="enabled" in at most one of the
>   buses, and as disabled everywhere else

We shouldn't need to do that. We can just let the driver check whether
the master provides the necessary capabilities to efficiently
communicate with the device, and if it does not just return -ENOTSUPP
in the ->probe() function. This way you'll have a device, but not
driver controlling it on one bus, and on the other bus, you'll have
another device (which points to the same physical device) this time
with a driver attached to it.

> - Use dynamic handover according to the bus protocol to
>   switch masters without having Linux even know that the
>   two buses are shared.

Yep.

> 
> That scenario would then fall completely into the "secondary
> master handover" category but require no special handling
> in the i3c layer beyond what we need for secondary masters
> that are managed by something outside of the kernel's
> score (a microcontroller, firmware, ...).

I definitely agree that both options should work. It's just that having
X times the same physical device represented in Linux seemed like a bad
thing to me, but it's also not something I'm strongly opposed to.

Regards,

Boris