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From: "Martinez, Ricardo" <ricardo.martinez@linux.intel.com>
To: Jakub Kicinski <kuba@kernel.org>,
	Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: netdev@vger.kernel.org, linux-wireless@vger.kernel.org,
	"David Miller" <davem@davemloft.net>,
	"Johannes Berg" <johannes@sipsolutions.net>,
	"Loic Poulain" <loic.poulain@linaro.org>,
	"M Chetan Kumar" <m.chetan.kumar@intel.com>,
	"Devegowda, Chandrashekar" <chandrashekar.devegowda@intel.com>,
	"Intel Corporation" <linuxwwan@intel.com>,
	chiranjeevi.rapolu@linux.intel.com,
	"Haijun Liu ( 刘海军)" <haijun.liu@mediatek.com>,
	"Hanania, Amir" <amir.hanania@intel.com>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Sharma, Dinesh" <dinesh.sharma@intel.com>,
	"Lee, Eliot" <eliot.lee@intel.com>,
	"Jarvinen, Ilpo Johannes" <ilpo.johannes.jarvinen@intel.com>,
	"Veleta, Moises" <moises.veleta@intel.com>,
	"Bossart, Pierre-louis" <pierre-louis.bossart@intel.com>,
	"Sethuraman, Muralidharan" <muralidharan.sethuraman@intel.com>,
	"Mishra, Soumya Prakash" <Soumya.Prakash.Mishra@intel.com>,
	"Kancharla, Sreehari" <sreehari.kancharla@intel.com>,
	"Sahu, Madhusmita" <madhusmita.sahu@intel.com>
Subject: Re: [PATCH net-next v8 02/14] net: skb: introduce skb_data_area_size()
Date: Mon, 9 May 2022 14:37:26 -0700	[thread overview]
Message-ID: <c4892d58-9d97-8fd7-800d-8433181cbda5@linux.intel.com> (raw)
In-Reply-To: <20220509125008.6d1c3b9b@kernel.org>


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.

  reply	other threads:[~2022-05-09 21:37 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-06 18:12 [PATCH net-next v8 00/14] net: wwan: t7xx: PCIe driver for MediaTek M.2 modem Ricardo Martinez
2022-05-06 18:12 ` [PATCH net-next v8 01/14] list: Add list_next_entry_circular() and list_prev_entry_circular() Ricardo Martinez
2022-05-06 18:12 ` [PATCH net-next v8 02/14] net: skb: introduce skb_data_area_size() Ricardo Martinez
2022-05-09 16:49   ` Jakub Kicinski
2022-05-09 18:34     ` Sergey Ryazanov
2022-05-09 19:50       ` Jakub Kicinski
2022-05-09 21:37         ` Martinez, Ricardo [this message]
2022-05-10 10:01           ` Andy Shevchenko
2022-05-10 13:37         ` Sergey Ryazanov
2022-05-06 18:12 ` [PATCH net-next v8 03/14] net: wwan: t7xx: Add control DMA interface Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 04/14] net: wwan: t7xx: Add core components Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 05/14] net: wwan: t7xx: Add port proxy infrastructure Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 06/14] net: wwan: t7xx: Add control port Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 07/14] net: wwan: t7xx: Add AT and MBIM WWAN ports Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 08/14] net: wwan: t7xx: Data path HW layer Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 09/14] net: wwan: t7xx: Add data path interface Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 10/14] net: wwan: t7xx: Add WWAN network interface Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 11/14] net: wwan: t7xx: Introduce power management Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 12/14] net: wwan: t7xx: Runtime PM Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 13/14] net: wwan: t7xx: Device deep sleep lock/unlock Ricardo Martinez
2022-05-06 18:13 ` [PATCH net-next v8 14/14] net: wwan: t7xx: Add maintainers and documentation Ricardo Martinez
2022-05-09 11:00 ` [PATCH net-next v8 00/14] net: wwan: t7xx: PCIe driver for MediaTek M.2 modem patchwork-bot+netdevbpf

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