netdev - Re: [PATCH net] netdevice: Prefer NETIF_F_HW

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Date:   Thu, 20 Apr 2017 11:20:19 -0400
From:   Vlad Yasevich <vyasevic@...hat.com>
To:     Michal Kubecek <mkubecek@...e.cz>,
        Vladislav Yasevich <vyasevich@...il.com>
Cc:     netdev@...r.kernel.org, tom@...bertland.com
Subject: Re: [PATCH net] netdevice: Prefer NETIF_F_HW_CSUM when intersecting
 features

On 04/20/2017 02:13 AM, Michal Kubecek wrote:
> On Wed, Apr 19, 2017 at 09:12:23PM -0400, Vladislav Yasevich wrote:
>> While hardware device use either NETIF_F_(IP|IPV6)_CSUM or
>> NETIF_F_HW_CSUM, all of the software devices use HW_CSUM.
>> This results in an interesting situation when the software
>> device is configured on top of hw device using (IP|IPV6)_CSUM.
>> In this situation, the user can't turn off checksum offloading
>> features on the software device.
>>
>> This patch resolves that by prefering the NETIF_F_HW_CSUM setting
>> when computing a feature intersect.
> 
> The reasoning makes sense to me but perhaps we should rename the helper
> then to make it obvious that it doesn't work like intersection anymore
> (not even in the logical sense).

It's still an intersect in the end since we return (f1 & f2).  We are just
munging checksum so that we get a HW_CSUM in the end if one of the devices supported
it.

> 
>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>> index 97456b25..3d811c1 100644
>> --- a/include/linux/netdevice.h
>> +++ b/include/linux/netdevice.h
>> @@ -4019,9 +4019,9 @@ static inline netdev_features_t netdev_intersect_features(netdev_features_t f1,
>>  {
>>  	if ((f1 ^ f2) & NETIF_F_HW_CSUM) {
>>  		if (f1 & NETIF_F_HW_CSUM)
>> -			f1 |= (NETIF_F_IP_CSUM|NETIF_F_IPV6_CSUM);
>> +			f2 |= NETIF_F_HW_CSUM;
>>  		else
>> -			f2 |= (NETIF_F_IP_CSUM|NETIF_F_IPV6_CSUM);
>> +			f1 |= NETIF_F_HW_CSUM;
>>  	}
>>  
>>  	return f1 & f2;
> 
> I hope I didn't miss something but it seems we can avoid nested ifs now:
> 
>  	if ((f1 ^ f2) & NETIF_F_HW_CSUM) {
> 		f1 |= NETIF_F_HW_CSUM;
> 		f2 |= NETIF_F_HW_CSUM;
>  	}
> 

Yes, we can do that.  We could have done something like this before as well, I suppose.

Thanks
-vlad

> Michal Kubecek
>