Re: [PATCH 0/2] hw/i2c: Adds pca954x i2c mux switch device

[Corey Minyard]

On Tue, Apr 06, 2021 at 08:41:50AM -0700, Patrick Venture wrote:
> On Mon, Apr 5, 2021 at 12:58 PM Corey Minyard <cminyard@mvista.com> wrote:
> >
> > On Sat, Apr 03, 2021 at 03:28:08PM -0700, Patrick Venture wrote:
> > > The i2c mux device pca954x implements two devices:
> > >  - the pca9546 and pca9548.
> > >
> > > Patrick Venture (2):
> > >   hw/i2c/core: add reachable state boolean
> > >   hw/i2c: add pca954x i2c-mux switch
> >
> > Looking this over, the code looks good, but I have a few general
> > questions:
> >
> > * Can you register the same slave address on different channels?  That's
> >   something you could do with real hardware and might be required at
> >   some time.  It looks like to me that you can't with this patch set,
> >   but maybe I'm missing something.
> 
> If I understand the hardware's implementation properly you can have
> collisions, and this allows for collisions.  I'm not sure what you
> mean by having both accessible.  For instance, on hardware you can
> have a switch with N channels, and on two of the channels there is an
> eeprom at 50.  But you're unable to talk to both eeproms at the same
> time, because the addresses collide -- so how would the hardware know
> which you're talking to?  My understanding of the behavior in this
> collision case is that it just talks to the first one that responds
> and can lead to unexpected things.

I wasn't talking about the collision case, I was talking about two
devices at the same address on two different channels.  (In a collision,
BTW, both devices will generaly be active and you will get undefined
results.)

My understanding of what you are doing, and I may be wrong, is that you
are adding the devices to the main bus and using an enable/disable to
turn the devices on/off depending on which channel is enabled.

It does look like you can add multiple devices to the same bus at the
same address, so I do think that works.

> 
> There is a board, the quanta-q71l where we had to set the
> idle-disconnect because there were two muxes on the same bus, with
> conflicting addresses, and so we had to use idle disconnect explicitly
> to make the software happy talking to the hardware -- not ideal as
> having two devices behind different channels, but ultimately it's the
> same idea because the devices are conflicting.
> 
> >
> > * Can you add devices to the secondary I2C busses on the mux using the
> >   standard QEMU device model, or is the function call required?
> 
> I added the function call because I didn't see a clean way to bridge
> the issue as well as, the quasi-arbitrary bus numbering used by the
> kernel isn't how the hardware truly behaves, and my goal was to
> implement closer to the hardware.  I thought about adding an I2cBus to
> the device and then you'd be able to access it, but wasn't sure of a
> nice clean way to plumb that through -- I considered adding/removing
> devices from the parent i2c bus instead of the boolean reachable, but
> that seemed way less clean - although do-able.

The only way I can think of with the method that you are using would be
to add a mux and channel to the i2c device, but that's not very natural.

The patch I did implements it by plumbing through, like you say.  It's a
little bit of a hack, but not too bad.

> 
> >
> > I ask because I did a pca9540 and pca9541 device, but I've never
> > submitted it because I didn't think it would ever be needed.  It takes a
> > different tack on the problem; it creates the secondary busses as
> > standard QEMU I2C busses and bridges them.  You can see it at
> >
> >    github.com:cminyard/qemu.git master-i2c-rebase
> >
> 
> I'll have to take a look at your approach, but the idea that it
> wouldn't be needed sounds bizarre to me as nearly all BMC-based qemu
> boards leverage i2c muxes to handle their PCIe slot i2c routing.

Yeah, I don't work in that world :).  I can see the need there, and
nobody has asked up til now.  I wish I had pushed it in earlier, then
your job would have been a lot easier.

-corey

> 
> > If you design can do the things I ask, then it's better.  If not, then
> > I'm not sure.
> >
> > -corey
> >
> > >
> > >  MAINTAINERS                      |   6 +
> > >  hw/i2c/Kconfig                   |   4 +
> > >  hw/i2c/core.c                    |   6 +
> > >  hw/i2c/i2c_mux_pca954x.c         | 182 +++++++++++++++++++++++++++++++
> > >  hw/i2c/meson.build               |   1 +
> > >  hw/i2c/trace-events              |   5 +
> > >  include/hw/i2c/i2c.h             |   3 +
> > >  include/hw/i2c/i2c_mux_pca954x.h |  60 ++++++++++
> > >  8 files changed, 267 insertions(+)
> > >  create mode 100644 hw/i2c/i2c_mux_pca954x.c
> > >  create mode 100644 include/hw/i2c/i2c_mux_pca954x.h
> > >
> > > --
> > > 2.31.0.208.g409f899ff0-goog
> > >