On Mon, 9 May 2022 21:34:58 +0300 Sergey Ryazanov wrote:
> >> +static inline unsigned int skb_data_area_size(struct sk_buff *skb)
> >> +{
> >> + return skb_end_pointer(skb) - skb->data;
> >> +}
> >
> > Not a great name, skb->data_len is the length of paged data.
> > There is no such thing as "data area", data is just a pointer
> > somewhere into skb->head.
>
> What name would you recommend for this helper?
We are assuming that skb->data is where we want to start to write
to so we own the skb. If it's a fresh skb skb->data == skb->tail.
If it's not fresh but recycled - skb->data is presumably reset
correctly, and then skb_reset_tail_pointer() has to be called. Either
way we give HW empty skbs, tailroom is an existing concept we can use.
> > Why do you need this? Why can't you use the size you passed
> > to the dev_alloc_skb() like everyone else?
>
> It was I who suggested to Ricardo to make this helper a common
> function [1]. So let me begin the discussion, perhaps Ricardo will add
> to my thoughts as the driver author.
>
> There are many other places where authors do the same but without a
> helper function:
>
> [...]
>
> There are at least two reasons to evaluate the linear data size from skb:
> 1) it is difficult to access the same variable that contains a size
> during skb allocation (consider skb in a queue);
Usually all the Rx skbs on the queue are equally sized so no need to
save the length per buffer, but perhaps that's not universally true.
> 2) you may not have access to an allocation size because a driver does
> not allocate that skb (consider a xmit path).
On Tx you should only map the data that's actually populated, for that
we have skb_headlen().
> Eventually you found themself in a position where you need to carry
> additional info along with skb. But, on the other hand, you can simply
> calculate this value from the skb pointers themselves.
>
> 1. https://lore.kernel.org/netdev/CAHNKnsTr3aq1sgHnZQFL7-0uHMp3Wt4PMhVgTMSAiiXT=8p35A@xxxxxxxxxxxxxx/
Fair enough, I didn't know more drivers are doing this.
We have two cases:
- for Tx - drivers should use skb_headlen();
- for Rx - I presume we are either dealing with fresh or correctly
reset skbs, so we can use skb_tailroom().
