linux-kernel - Re: [PATCH 0/1] i2c: imc: Add support for Intel iMC SMBus host controller.

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Date:   Sun,  1 Mar 2020 11:02:10 -0800 (PST)
From:   Stefan Schaeckeler <schaecsn@....net>
To:     luto@...nel.org
CC:     linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] i2c: imc: Add support for Intel iMC SMBus host controller.

Hello Any,

> > This patch is based on Andy Lutomirski's iMC SMBus driver patch-set
> > https://lkml.org/lkml/2016/4/28/926. It never made it into the kernel. I hope
> > this rewrite will:
> >
> >
> > Overview
> >
> > Modern Intel memory controllers host an SMBus controller and connection to
> > DIMMs and their thermal sensors. The memory controller firmware has three modes
> > of operation: Closed Loop Thermal Throttling (CLTT), Open Loop Thermal
> > Throttling (OLTT) and none.
> >
> > - CLTT: The memory controller firmware is periodically accessing the DIMM
> >   temperature sensor over the SMBus.
> >
>
>
> I think this is great!  One question, though: what happens if the
> system is in CLTT mode but you disable CLTT and claim the bus for too
> long?  For example, if there's an infinite loop or other lockup which
> you have the tsod polling interval set to 0?  Does the system catch
> fire or does the system do something intelligent like temporarily
> switching to open loop?

I don't know. Most likely, the current memory throttling rate will be kept.
That might not be enough for the forthcoming workload and, ehm, the system may
catch fire.

I assume our use-case is the most common use-case for this driver: our embedded
system comes with its own environmental management software. It monitors, among
other sensor values, the DIMM temperatures and takes action on abnormal values.
If one is concerned about your scenario, then the environmental management
software needs to consider blocked reads on the sysfs node as a worst case
scenario and reboot the system.

Nothing can really go wrong while the polling interval is set to 0, though:

- reading and setting pci configuration space registers.
- calling dev_err, dev_warn and alike.
- usleep_range(131,140) and up to 20 udelay(9).

What is not clear to me is what if imc_smbus_xfer() is executing while the
driver is rmmod-ed. Defensively, I set in the driver's remove function the
tsod_polling_interval back to its original value.

~ Stefan