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

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Message-ID: <c4892d58-9d97-8fd7-800d-8433181cbda5@linux.intel.com>
Date:   Mon, 9 May 2022 14:37:26 -0700
From:   "Martinez, Ricardo" <ricardo.martinez@...ux.intel.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Sergey Ryazanov <ryazanov.s.a@...il.com>
Cc:     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 5/9/2022 12:50 PM, Jakub Kicinski wrote:
> 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().
Thanks for the information, looks like indeed we can avoid the helper in 
t7xx driver by following those guidelines.