linux-kernel - Re: [PATCH v2 0/2] ic2: mux: pca9541: add delayed-release support

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Date:   Mon, 28 Feb 2022 22:54:09 +0100
From:   Peter Rosin <peda@...ntia.se>
To:     Zev Weiss <zev@...ilderbeest.net>, linux-i2c@...r.kernel.org,
        Guenter Roeck <linux@...ck-us.net>,
        Rob Herring <robh+dt@...nel.org>
Cc:     openbmc@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v2 0/2] ic2: mux: pca9541: add delayed-release support

On 2022-02-01 01:18, Zev Weiss wrote:
> Hello,
> 
> This series adds support for a new pca9541 device-tree property
> ("release-delay-us"), which delays releasing ownership of the bus
> after a transaction for a configurable duration, anticipating that
> another transaction may follow shortly.  By avoiding a
> release/reacquisition between transactions, this can provide a
> substantial performance improvement for back-to-back operations -- on
> a Delta AHE-50DC (ASPEED AST1250) system running OpenBMC with dozens
> of LM25066 PMICs on PCA9541-arbitrated busses, a setting of 10000 (10
> ms) reduces the median latency the psusensor daemon's hwmon sysfs file
> reads from 2.28 ms to 0.99 ms (a 57% improvement).

Hi!

Sorry for the late reply. It seems I'm forever swamped...

There is a risk with this scheme. If you have two (or more) of these
chips on the same bus, and there are clients behind these two chips
that have the same address, accesses to one of the clients might
"leak through" the other arb to an unexpected client when its arb
is in it's release-delay state.

In other words, it is no coincidence that the segment lock is held
over the whole acquire-access-release cycle.

Sure, you can always say "don't add a release-delay when ...", but I
see no such documentation.

I have no access to this HW either.

Cheers,
Peter