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

[Vivien Didelot]

Hi Rasmus, Andrew,

On Wed, 29 May 2019 06:42:53 +0000, Rasmus Villemoes 
<rasmus.villem...@prevas.dk> wrote:
> On 28/05/2019 15.44, Andrew Lunn wrote:
> > 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.
> 
> Will do.
> 
> >> 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;
> 
> Why? It's odd to have the u32 "high" sometimes represent the high 32
> bits, sometimes the third 16 bits. It would make it harder to support an
> 8-byte STATS_TYPE_PORT statistic. I think the code is much cleaner if
> each case is just responsible for providing the upper/lower 32 bits,
> then have the common case combine them; . It's just that in the
> STATS_TYPE_BANK cases, the 32 bits are assembled from two 16 bit values
> by a helper (mv88e6xxx_g1_stats_read), while it is "open-coded" in the
> first case.

Here "low" and "high" are u32 and refer to the upper and lower halves
of the u64 returned value. For a 4-byte STATS_TYPE_PORT value, the
second read refers to the bits 31:16, thus belongs to (the upper half
of) the lower half of the 64-bit returned value, i.e. "low".

The current return value is correct, as well as your patch.


Thanks,
Vivien