Re: [PATCH] net: dsa: mv88e6xxx: fix handling of upper half of STATS_TYPE_PORT

[Andrew Lunn]

On Tue, May 28, 2019 at 01:17:10PM +0000, Rasmus Villemoes wrote:
> Currently, the upper half of a 4-byte STATS_TYPE_PORT statistic ends
> up in bits 47:32 of the return value, instead of bits 31:16 as they
> should.
> 
> Signed-off-by: Rasmus Villemoes <rasmus.villem...@prevas.dk>

Hi Rasmus

Please include a Fixes tag, to indicate where the problem was
introduced. In this case, i think it was:

Fixes: 6e46e2d821bb ("net: dsa: mv88e6xxx: Fix u64 statistics")

And set the Subject to [PATCH net] to indicate this should be applied
to the net tree.

> ---
> I also noticed that it's a bit inconsistent that we return U64_MAX if
> there's a read error in STATS_TYPE_PORT, while
> mv88e6xxx_g1_stats_read() returns 0 in case of a read error. In
> practice, register reads probably never fail so it doesn't matter.
> 
>  drivers/net/dsa/mv88e6xxx/chip.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/dsa/mv88e6xxx/chip.c 
> b/drivers/net/dsa/mv88e6xxx/chip.c
> index 370434bdbdab..317553d2cb21 100644
> --- a/drivers/net/dsa/mv88e6xxx/chip.c
> +++ b/drivers/net/dsa/mv88e6xxx/chip.c
> @@ -785,7 +785,7 @@ static uint64_t _mv88e6xxx_get_ethtool_stat(struct 
> mv88e6xxx_chip *chip,
>                       err = mv88e6xxx_port_read(chip, port, s->reg + 1, &reg);
>                       if (err)
>                               return U64_MAX;
> -                     high = reg;
> +                     low |= ((u32)reg) << 16;
>               }
>               break;
>       case STATS_TYPE_BANK1:

What i don't like about this is how the function finishes:

        }
        value = (((u64)high) << 32) | low;
        return value;
}

A better fix might be

-               break
+               value = (((u64)high) << 16 | low;
+               return value;

        Andrew