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

[Jakub Kicinski <kuba@kernel.org>]

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.