linux-kernel - Re: [PATCH] hwmon: (peci/dimmtemp) Do not provide fake thresholds data

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Message-ID: <Z5faC6M2MUj8KYoB@home.paul.comp>
Date: Mon, 27 Jan 2025 22:10:03 +0300
From: Paul Fertser <fercerpav@...il.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: "Winiarska, Iwona" <iwona.winiarska@...el.com>,
        "jae.hyun.yoo@...ux.intel.com" <jae.hyun.yoo@...ux.intel.com>,
        "Rudolph, Patrick" <patrick.rudolph@...ements.com>,
        "pierre-louis.bossart@...ux.dev" <pierre-louis.bossart@...ux.dev>,
        "Solanki, Naresh" <naresh.solanki@...ements.com>,
        "jdelvare@...e.com" <jdelvare@...e.com>,
        "fr0st61te@...il.com" <fr0st61te@...il.com>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "openbmc@...ts.ozlabs.org" <openbmc@...ts.ozlabs.org>,
        "joel@....id.au" <joel@....id.au>
Subject: Re: [PATCH] hwmon: (peci/dimmtemp) Do not provide fake thresholds
 data

On Mon, Jan 27, 2025 at 10:39:44AM -0800, Guenter Roeck wrote:
> On 1/27/25 10:30, Paul Fertser wrote:
> > Hi Guenter,
> > 
> > On Mon, Jan 27, 2025 at 09:29:39AM -0800, Guenter Roeck wrote:
> > > On 1/27/25 08:40, Winiarska, Iwona wrote:
> > > > On Thu, 2025-01-23 at 15:20 +0300, Paul Fertser wrote:
> > > > > When an Icelake or Sapphire Rapids CPU isn't providing the maximum and
> > > > > critical thresholds for particular DIMM the driver should return an
> > > > > error to the userspace instead of giving it stale (best case) or wrong
> > > > > (the structure contains all zeros after kzalloc() call) data.
> > > > > 
> > > > > The issue can be reproduced by binding the peci driver while the host is
> > > > > fully booted and idle, this makes PECI interaction unreliable enough.
> > > > > 
> > > > > Fixes: 73bc1b885dae ("hwmon: peci: Add dimmtemp driver")
> > > > > Fixes: 621995b6d795 ("hwmon: (peci/dimmtemp) Add Sapphire Rapids support")
> > > > > Cc: stable@...r.kernel.org
> > > > > Signed-off-by: Paul Fertser <fercerpav@...il.com>
> > > > 
> > > > Hi!
> > > > 
> > > > Thank you for the patch.
> > > > Did you have a chance to test it with OpenBMC dbus-sensors?
> > > > In general, the change looks okay to me, but since it modifies the behavior
> > > > (applications will need to handle this, and returning an error will happen more
> > > > often) we need to confirm that it does not cause any regressions for userspace.
> > > > 
> > > 
> > > I would also like to understand if the error is temporary or permanent.
> > > If it is permanent, the attributes should not be created in the first
> > > place. It does not make sense to have limit attributes which always report
> > > -ENODATA.
> > 
> > The error is temporary. The underlying reason is that when host CPUs
> > go to deep enough idle sleep state (probably C6) they stop responding
> > to PECI requests from BMC. Once something starts running the CPU
> > leaves C6 and starts responding and all the temperature data
> > (including the thresholds) becomes available again.
> > 
> 
> Thanks.
> 
> Next question: Is there evidence that the thresholds change while the CPU
> is in a deep sleep state (or, in other words, that they are indeed stale) ?
> Because if not it would be (much) better to only report -ENODATA if the
> thresholds are uninitialized, and it would be even better than that if the
> limits are read during initialization (and not updated at all) if they do
> not change dynamically.

>From BMC point of view when getting a timeout there is little
difference between the host not answering being in idle deep sleep
state and between host being completely powered off. Now I can imagine
a server system where BMC keeps running and the server has its DIMMs
physically changed to a different model with different threshold.

Whether it's realistic scenario and whether it's worth caching the
thresholds in the kernel I hope Iwona can clarify. In my current
opinion the added complexity isn't worth it, the PECI operation needs
to be reliable enough anyway for BMC to monitor at least the CPU
temperatures once a second to feed this essential data to the cooling
fans control loop. And if we can read CPU temperatures we can also
read DIMM thresholds when we need them and worse case retry a few
times while starting up the daemon.

-- 
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