netdev - Re: [PATCH net-next v8 02/14] net: skb: introduce skb_data_area

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Date:   Mon, 9 May 2022 12:50:08 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Sergey Ryazanov <ryazanov.s.a@...il.com>
Cc:     Ricardo Martinez <ricardo.martinez@...ux.intel.com>,
        netdev@...r.kernel.org, linux-wireless@...r.kernel.org,
        David Miller <davem@...emloft.net>,
        Johannes Berg <johannes@...solutions.net>,
        Loic Poulain <loic.poulain@...aro.org>,
        M Chetan Kumar <m.chetan.kumar@...el.com>,
        "Devegowda, Chandrashekar" <chandrashekar.devegowda@...el.com>,
        Intel Corporation <linuxwwan@...el.com>,
        chiranjeevi.rapolu@...ux.intel.com,
        "Haijun Liu ( 刘海军) " <haijun.liu@...iatek.com>,
        "Hanania, Amir" <amir.hanania@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Sharma, Dinesh" <dinesh.sharma@...el.com>,
        "Lee, Eliot" <eliot.lee@...el.com>,
        "Jarvinen, Ilpo Johannes" <ilpo.johannes.jarvinen@...el.com>,
        "Veleta, Moises" <moises.veleta@...el.com>,
        "Bossart, Pierre-louis" <pierre-louis.bossart@...el.com>,
        "Sethuraman, Muralidharan" <muralidharan.sethuraman@...el.com>,
        "Mishra, Soumya Prakash" <Soumya.Prakash.Mishra@...el.com>,
        "Kancharla, Sreehari" <sreehari.kancharla@...el.com>,
        "Sahu, Madhusmita" <madhusmita.sahu@...el.com>
Subject: Re: [PATCH net-next v8 02/14] net: skb: introduce
 skb_data_area_size()

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@mail.gmail.com/

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().