Re: [PATCH v8 3/3] net: ax88796c: ASIX AX88796C SPI Ethernet Adapter Driver

[Jakub Kicinski]

On Thu, 17 Dec 2020 12:46:57 +0100 Lukasz Stelmach wrote:
> > to the correct values so the stack pre-allocates the needed spaces,
> > when it can.  
> 
> Yes, I fonud these. However, I am not sure setting needed_tailroom has
> any effect. In many places where alloc_skb() is called needed_headrom
> and hard_header_len are refered to via the LL_RESERVED_SPACE macro. But
> the macro does not refer to needed_tailroom. Once (f5184d267c1a ("net:
> Allow netdevices to specify needed head/tailroom") there was
> LL_ALLOCATED_SPACE macro, but but it was removed in 56c978f1da1f ("net:
> Remove LL_ALLOCATED_SPACE"). And now only some protocols refer to
> needet_tailroom.

Yeah, tailroom is used a lot less often. Only really crappy HW requires
it.

> BTW. What is hard_header_len for? Is it the length of the link layer
> header? Considering "my" hardware requires some headers with each
> packet, I find hard_headr_len name a bit confusing.

Yup, L2 headers, not hardware. Not sure why "hard" was chosen, that must
have happened way back.