Re: [patch net / RFC] net: fec: increase frame size limitation to actually available buffer

[Toshiaki Makita]

On 2016/11/30 15:36, Nikita Yushchenko wrote:
>> But I think it is not necessary since the driver don't support jumbo frame.
> 
> Hardcoded 1522 raises two separate issues.
> 
> (1) When DSA is in use, frames processed by FEC chip contain DSA tag and
> thus can be larger than hardcoded limit of 1522. This issue is not
> FEC-specific, any driver that hardcodes maximum frame size to 1522 (many
> do) will have this issue if used with DSA.
> 
> Clean solution for this must take into account that difference between
> MTU and max frame size is no longer known at compile time. Actually this
> is the case even without DSA, due to VLANs: max frame size is (MTU + 18)
> without VLANs, but (MTU + 22) with VLANs. However currently drivers tend
> to ignore this and hardcode 22.  With DSA, 22 is not enough, need to add
> switch-specific tag size to that.
> 
> Not yet sure how to handle this. DSA-specific API to find out tag size
> could be added, but generic solution should handle all cases of dynamic
> difference between MTU and max frame size, not only DSA.

BTW I'm trying to introduce envelope frames to solve this kind of problems.
http://marc.info/?t=147496691500005&r=1&w=2
http://marc.info/?t=147496691500003&r=1&w=2
http://marc.info/?t=147496691500002&r=1&w=2
http://marc.info/?t=147496691500004&r=1&w=2
http://marc.info/?t=147496691500001&r=1&w=2

It needs jumbo frame support of NICs though.

Regards,
Toshiaki Makita