netdev - Re: [PATCH net-next 1/1] net: dsa: mv88e6xxx: allow reading FID when handling ATU violations

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Message-ID: <20220707174358.3b1b804a@kernel.org>
Date:   Thu, 7 Jul 2022 17:43:58 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Hans Schultz <netdev@...io-technology.com>
Cc:     Vladimir Oltean <olteanv@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Shuah Khan <shuah@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Ido Schimmel <idosch@...dia.com>, linux-kernel@...r.kernel.org,
        bridge@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next 1/1] net: dsa: mv88e6xxx: allow reading FID
 when handling ATU violations

On Thu, 7 Jul 2022 13:28:36 +0300 Vladimir Oltean wrote:
> Make no mistake, the existing code doesn't disallow reading back the FID
> during an ATU Get/Clear Violation operation, and your patch isn't
> "allowing" something that wasn't disallowed.
> 
> The documentation for the ATU FID register says that its contents is
> ignored before the operation starts, and it contains the returned ATU
> entry's FID after the operation completes.
> 
> So the change simply says: don't bother to write the ATU FID register
> with zero, it doesn't matter what this contains. This is probably true,
> but the patch needs to do what's written on the box.
> 
> Please note that this only even matters at all for switches with
> mv88e6xxx_num_databases(chip) > 256, where MV88E6352_G1_ATU_FID is a
> dedicated register which this patch avoids writing. For other switches,
> the FID is embedded within MV88E6XXX_G1_ATU_CTL or MV88E6XXX_G1_ATU_OP.
> So _practically_, for those switches, you are still emitting the
> GET_CLR_VIOLATION ATU op with a FID of 0 whether you like it or not, and
> this patch introduces a (most likely irrelevant) discrepancy between the
> access methods for various switches.
> 
> Please note that this observation is relevant for your future changes to
> read back the FID too. As I said here:
> https://patchwork.kernel.org/project/netdevbpf/patch/20220524152144.40527-4-schultz.hans+netdev@gmail.com/#24912482
> you can't just assume that the FID lies within the MV88E6352_G1_ATU_FID
> register, just look at the way it is packed within mv88e6xxx_g1_atu_op().
> You'll need to unpack it in the same way.

I reckon it'll be useful to render some of this info into the commit
message and adjust the subject so marking Changes Requested.