On Tue, Apr 06, 2021 at 08:41:50AM -0700, Patrick Venture wrote: > On Mon, Apr 5, 2021 at 12:58 PM Corey Minyard <[email protected]> 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 > > >
