Configuring device with I2C mux

[Alex Qiu]

Hi Milton,

Yes, the child buses are traceable in the sysfs, but actually my question
is, does entity-manager supports tracing child buses through the sysfs? If
it does, how shall I write the JSON config to take advantage of this
feature?

For example, we have a I2C device sitting at the same level as the mux,
configured as {"Address": "0x40", "Bus": "$bus"}, and now I don't find the
way to add or subtract a certain number to $bus to find a child bus. Do we
have a feature with syntax like {"Address": "0x50", "Bus":
"$bus.mux.0x70.channel.1"} to configure devices on child buses, with mux
sitting on 0x70 of the parent bus?

Thank you!

- Alex Qiu


On Wed, Jan 8, 2020 at 6:43 AM Milton Miller II <miltonm at us.ibm.com> wrote:

> About 01/07/2020 07:14PM in some timezone, Alex Qiu wrote:
> >Hi folks,
> >
> >We have a JSON file configuring I2C devices of a PCIe card which has
> >an 8-channel I2C mux for entity-manager. After we properly configure
> >the mux of the card, we get 8 new I2C buses in sysfs; however, we
> >don't find an arithmetic way to describe the relationship between the
> >new buses and their parent buses.
> >
> >For example, we have a mux on physical BMC I2C bus i2c-8 spawning a
> >couple of child buses including i2c-24 and i2c-26, which are the
> >buses for each PCIe card. The mux on i2c-24 PCIe card spawned 8 buses
> >from i2c-70 to i2c-77, and the mux on i2c-26 PCIe card spawned ones
> >from i2c-62 to i2c-69. We have I2C devices on i2c-76 and i2c-68 needs
> >to be configured together with the PCIe card on i2c-24 and i2c-26
> >separately.
> >
> >How do we implement this in the JSON file? If that's not possible to
> >do it in JSON merely, what's the suggested approach to implement this
> >feature? Device tree? Implement code in entity-manager to walk
> >through the I2C sysfs?
>
>
> I would expect the i2c buses provied by the mux to appear in sysfs
> under the hierarchy of the parent bus.  Explore the sysfs directory
> structure, I'm making this up, but I've now peeked at i2c-mux.c and
> have a better chance getting reality.
>
>
> /sys/bus/i2c/devices/i2c-8 -> /sys/devices/platform/ahb-432000/i2c-8
>
> /sys/bus/i2c/devices/i2c-24 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24
> /sys/bus/i2c/devices/i2c-24 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-25
> /sys/bus/i2c/devices/i2c-24 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26
>
> /sys/bus/i2c/devices/i2c-62 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-62
> /sys/bus/i2c/devices/i2c-63 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-63
> /sys/bus/i2c/devices/i2c-64 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-64
> /sys/bus/i2c/devices/i2c-65 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-65
> /sys/bus/i2c/devices/i2c-66 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-66
> /sys/bus/i2c/devices/i2c-67 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-67
> /sys/bus/i2c/devices/i2c-68 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-68
> /sys/bus/i2c/devices/i2c-69 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-69
> /sys/bus/i2c/devices/i2c-70 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-70
> /sys/bus/i2c/devices/i2c-71 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-71
> /sys/bus/i2c/devices/i2c-72 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-72
> /sys/bus/i2c/devices/i2c-73 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-73
> /sys/bus/i2c/devices/i2c-74 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-74
> /sys/bus/i2c/devices/i2c-75 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-75
> /sys/bus/i2c/devices/i2c-76 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-76
> /sys/bus/i2c/devices/i2c-77 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-77
>
> /sys/bus/i2c/devices/i2c-24/mux_device ->
> /sys/devices/platform/ahb-432000/i2c-8/8-52
> /sys/bus/i2c/devices/i2c-25/mux_device ->
> /sys/devices/platform/ahb-432000/i2c-8/8-52
> /sys/bus/i2c/devices/i2c-26/mux_device ->
> /sys/devices/platform/ahb-432000/i2c-8/8-52
>
> /sys/bus/i2c/devices/8-52/channel-0 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24
> /sys/bus/i2c/devices/8-52/channel-1 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-25
> /sys/bus/i2c/devices/8-52/channel-2 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26
>
> /sys/bus/i2c/devices/24-52/channel-0 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-62
> /sys/bus/i2c/devices/24-52/channel-1 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-63
> /sys/bus/i2c/devices/24-52/channel-2 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-64
> /sys/bus/i2c/devices/24-52/channel-3 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-65
> /sys/bus/i2c/devices/24-52/channel-4 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-66
> /sys/bus/i2c/devices/24-52/channel-5 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-67
> /sys/bus/i2c/devices/24-52/channel-6 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-68
> /sys/bus/i2c/devices/24-52/channel-7 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-24/i2c-69
> /sys/bus/i2c/devices/26-52/channel-0 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-70
> /sys/bus/i2c/devices/26-52/channel-1 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-71
> /sys/bus/i2c/devices/26-52/channel-2 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-72
> /sys/bus/i2c/devices/26-52/channel-3 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-73
> /sys/bus/i2c/devices/26-52/channel-4 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-74
> /sys/bus/i2c/devices/26-52/channel-5 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-75
> /sys/bus/i2c/devices/26-52/channel-6 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-76
> /sys/bus/i2c/devices/26-52/channel-7 ->
> /sys/devices/platform/ahb-432000/i2c-8/i2c-26/i2c-77
>
> The child bus will have a symlink "mux_device" pointing to the mux.
> The mux device node will have n symlinks "channel-1" where 1 is
> the channel id.
>
> Hmm, there is no apparent code to put a "mux_channel" attribute in
> the adapter node.  The adapter name attribute does include the channel
> id, but its long descriptive text.  Regardless, you should be able to
> find the buses from the mux channel-3 symlinks.
>
> Note: I haven't looked at the JSON to see how much it will change once
> your find the bus.  I'm guessing you will have to generate it by taking
> a template and substituting the bus numbers you find to make the slots
> common, alhtough you could just go from the 8-52/channel-3 symlink to
> the adapter bus (the device name will still have the bus number in it).
>
> I hope this helps you,
>
> milton
>
>
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