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

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Message-ID: <f8a4f54a90efa545cac1ff2cdbde78c7@kapio-technology.com>
Date:   Fri, 08 Jul 2022 09:12:46 +0200
From:   netdev@...io-technology.com
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, 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 2022-07-07 12:28, Vladimir Oltean wrote:
> On Wed, Jul 06, 2022 at 02:25:02PM +0200, Hans Schultz wrote:
>> For convenience the function mv88e6xxx_g1_atu_op() has been used to 
>> read
>> ATU violations, but the function has other purposes and does not 
>> enable
>> the possibility to read the FID when reading ATU violations.
>> 
>> The FID is needed to get hold of which VID was involved in the 
>> violation,
>> thus the need for future purposes to be able to read the FID.
> 
> 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.

It would only read 0 the way it worked. And I don't understand why
mv88e6xxx_g1_atu_op() writes the FID?

> 
> 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.

Writing 0 to the ATU fID register resulted in a read giving zero of 
course.

> 
> 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.

So I need a new function to read the FID that mimics 
mv88e6xxx_g1_atu_op()
as far as I understand?