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From: Ed Tanous <edtanous@google.com>
Date: Fri, 11 Feb 2022 12:20:05 -0800
Message-ID: <CAH2-KxCOOqoXmBLmR+=Jjrea1fouCOYOb4pGwZHwk5c=QJHeJg@mail.gmail.com>
Subject: Re: No dbus objects for phosphor-regulators
To: Shawn McCarney <shawnmm@linux.ibm.com>,
 Justin Ledford <justinledford@google.com>, 
 Gunnar Mills <gmills@linux.vnet.ibm.com>, Andrew Jeffery <andrew@aj.id.au>, 
 Adriana Kobylak <anoo@linux.ibm.com>, Peter Lundgren <peterlundgren@google.com>,
 Jason Ling <jasonling@google.com>
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 <openbmc-bounces+openbmc=archiver.kernel.org@lists.ozlabs.org>

On Fri, Feb 11, 2022 at 8:06 AM Shawn McCarney <shawnmm@linux.ibm.com> wrot=
e:
>
> On 2/11/2022 9:42 AM, Mike Jones wrote:
>
> I was mainly surprised because a conversation I had with Guenter, if I re=
member correctly, suggested that /dev/i2c calls from user space work with h=
wmon, because hwmon does not lock the i2c except when using it.
>
> So I assumed that in this case, it was the polling of hwmon that was just=
 keeping it locked enough to conflict with phosphor-regulators and then it =
gives up.
>
> With i2c-dev, you can specify I2C_SLAVE or I2C_SLAVE_FORCE when creating =
the connection.  I believe I2C_SLAVE will cause communication attempts to f=
ail if a driver is bound.  I2C_SLAVE_FORCE will still try to communicate.
>
> I wanted to take the more conservative approach, because there are scenar=
ios where interleaving communication can cause serious problems.  For examp=
le, if one PMBus regulator supports multiple rails, you have to set the PMB=
us PAGE register to the rail you are interested in.  Subsequent PMBus comma=
nds will affect that rail.  If phosphor-regulators and a driver are both co=
mmunicating with the regulator, they may both be setting PAGE and thus thei=
r subsequent commands are going to the wrong rail.
>
>
> I could just not use hwmon at all use phosphor-regulators for all telemet=
ry, but this seemed like more work.
>
> Sorry about that.  I think if you have already done the work of defining =
the regulator and a read sensors rule in your JSON file, adding a few more =
sensors is not much additional work though.  Let me know if you have questi=
ons.
>
> Also, I will need to figure out how to connect phosphor-regulators teleme=
try to Redfish and the WebUI. Are there examples of how to do that?
>
> It should just work with no additional effort as long as you define the f=
ollowing correctly in your JSON file:
>
> * The "inventory_path" property of your "chassis" object needs to match a=
 real chassis inventory item on D-Bus.
>
> * The "fru" property of your regulator "device" object needs to match a r=
eal inventory item on D-Bus.  As I mentioned earlier, normally if the regul=
ator is pluggable then this is the regulator itself.  Otherwise it is the l=
arger hardware item that contains it, such as a motherboard.  FRU is the ac=
ronym for Field Replaceable Unit, referring to a pluggable hardware item.
>
> phosphor-regulators will publish your sensors on D-Bus with the standard =
object paths and properties.  The two JSON properties above are used to def=
ine a "chassis" association and an "inventory_item" association on D-Bus th=
at is used by bmcweb.  This will cause your sensors to appear in Redfish an=
d the Web UI.
>
> Thanks,
>
> Shawn

This same topic is essentially being discussed in a design doc here:
https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/50509

I've CCed the relevant parties from that review on this thread, and I
don't have a strong opinion what forum we use to discuss this, but we
should ideally all be talking together on a common solution.