Re: [PATCH net-next v3 4/6] net: dsa: mv88e6xxx: simplify SERDES code for Topaz and Peridot

[Vivien Didelot <vivien.didelot@gmail.com>]

Hi Marek,

On Sun, 25 Aug 2019 18:36:09 +0200, Marek Behun <marek.behun@nic.cz> wrote:
> > Aren't you relying on -ENODEV as well?
> 
> Vivien, I am not relying o -ENODEV. I changed the serdes_get_lane
> semantics:
>  - previously:
>    - if port has a lane for current cmode, return given lane number
>    - otherwise return -ENODEV
>    - if other error occured during serdes_get_lane, return that error
>      (this never happened, because all implementations only need port
>      number and cmode, and cmode is cached, so no function was called
>      that could err)
>  - after this commit:
>    - if port has a lane for current cmode, return 0 and put lane number
>      into *lane
>    - otherwise return 0 and put -1 into *lane
>    - if error occured, return that error number
> 
> I removed the -ENODEV semantics for "no lane on port" event.
> There are two reasons for this:
>   1. once you requested lane number to be put into a place pointed to
>      by a pointer, rather than the return value, the code seemed better
>      to me (you may of course disagree, this is a personal opinion) when
>      I did:
>        if (err)
>            return err;
>        if (lane < 0)
>            return 0;
>      rather than
>        if (err == -ENODEV)
>            return 0;
>        if (err)
>            return err;

A single return path for invalid queries, eventually checking a specific
error, is always more idiomatic and better than checking two places which
could lead in mistakes as your previous patch did. So this is more readable:

    if (err)
        return err;

or:

    if (err && err != -ENODEV)
        return err;

or:

    if (err) {
        if (err = -ENODEV)
            err = 0;
        return err;
    }

>   2. some future implementation may actually need to call some MDIO
>      read/write functions, which may or may not return -ENODEV. That
>      could conflict with the -ENODEV returned when there is no lane.

The current code is already using -ENODEV to inform about "no lane for port",
even if it can be used by lower level functions, same as -EINVAL. That is fine.

So if you have to respin the series again, I would really prefer to see an
unsigned lane parameter, otherwise, fine...


Thanks,

	Vivien