Re: Configuring device with I2C mux

[James Feist <james.feist@linux.intel.com>]

On 1/8/20 9:25 AM, Alex Qiu wrote:
> 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?

Do you have a device sitting behind the mux? Generally in most our 
situations we have another FRU behind the mux, and we can use "$bus" to 
describe this.

Second option is you can name the mux channels, like this: 
https://github.com/openbmc/entity-manager/blob/01542d2af1b1f45335cc8813fffcd3ed07f22989/configurations/WFT%20Baseboard.json#L1023

And then in /dev/ you'll get symlinks like this:

root@intel-obmc:~# ls -al /dev/i2c-mux/M2_Mux/
drwxr-xr-x    2 root     root            80 Jan  1 00:01 .
drwxr-xr-x    9 root     root           180 Jan  1 00:01 ..
lrwxrwxrwx    1 root     root            11 Jan  1 00:01 M2Slot1 -> 
/dev/i2c-14
lrwxrwxrwx    1 root     root            11 Jan  1 00:01 M2Slot2 -> 
/dev/i2c-15

-James


> 
> Thank you!
> 
> - Alex Qiu
> 
> 
> On Wed, Jan 8, 2020 at 6:43 AM Milton Miller II <miltonm@us.ibm.com 
> <mailto:miltonm@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
>