linux-kernel - Re: [PATCH] net: dsa: mv88e6xxx: fix handling of upper half of STATS_TYPE

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Message-ID: <dc84827e-6bac-7a01-f998-609cfe9a33ec@prevas.dk>
Date:   Wed, 29 May 2019 06:42:53 +0000
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Andrew Lunn <andrew@...n.ch>
CC:     Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Rasmus Villemoes <Rasmus.Villemoes@...vas.se>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: dsa: mv88e6xxx: fix handling of upper half of
 STATS_TYPE_PORT

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.villemoes@...vas.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.

I'll resend my patch with the fixes tag (thanks for finding that; I had
already dug way too deep past that one) and fixed subject.

Rasmus


> 	Andrew	
> 


-- 
Rasmus Villemoes
Software Developer
Prevas A/S
Hedeager 3
DK-8200 Aarhus N
+45 51210274
rasmus.villemoes@...vas.dk
www.prevas.dk