linux-kernel - Re: [PATCH] wwan: core: Support slicing in port TX flow of WWAN subsystem

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Date:   Thu, 27 Oct 2022 09:19:42 +0800
From:   haozhe chang <haozhe.chang@...iatek.com>
To:     Loic Poulain <loic.poulain@...aro.org>
CC:     "chandrashekar.devegowda@...el.com" 
        <chandrashekar.devegowda@...el.com>,
        "linuxwwan@...el.com" <linuxwwan@...el.com>,
        "chiranjeevi.rapolu@...ux.intel.com" 
        <chiranjeevi.rapolu@...ux.intel.com>,
        Haijun Liu (刘海军) 
        <haijun.liu@...iatek.com>,
        "m.chetan.kumar@...ux.intel.com" <m.chetan.kumar@...ux.intel.com>,
        "ricardo.martinez@...ux.intel.com" <ricardo.martinez@...ux.intel.com>,
        "ryazanov.s.a@...il.com" <ryazanov.s.a@...il.com>,
        "johannes@...solutions.net" <johannes@...solutions.net>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Lambert Wang (王伟) 
        <Lambert.Wang@...iatek.com>,
        "Xiayu Zhang (张夏宇)" 
        <Xiayu.Zhang@...iatek.com>,
        "srv_heupstream@...iatek.com" <srv_heupstream@...iatek.com>
Subject: Re: [PATCH] wwan: core: Support slicing in port TX flow of WWAN
 subsystem

On Wed, 2022-10-26 at 22:27 +0800, Loic Poulain wrote:
> On Wed, 26 Oct 2022 at 13:45, haozhe chang <haozhe.chang@...iatek.com
> > wrote:
> > 
> > On Wed, 2022-10-26 at 15:28 +0800, Loic Poulain wrote:
> > > Hi Haozhe,
> > > 
> > > On Wed, 26 Oct 2022 at 03:16, <haozhe.chang@...iatek.com> wrote:
> > > > 
> > > > From: haozhe chang <haozhe.chang@...iatek.com>
> > > > 
> > > > wwan_port_fops_write inputs the SKB parameter to the TX
> > > > callback of
> > > > the WWAN device driver. However, the WWAN device (e.g., t7xx)
> > > > may
> > > > have an MTU less than the size of SKB, causing the TX buffer to
> > > > be
> > > > sliced and copied once more in the WWAN device driver.
> > > 
> > > The benefit of putting data in an skb is that it is easy to
> > > manipulate, so not sure why there is an additional copy in the
> > > first
> > > place. Isn't possible for the t7xx driver to consume the skb
> > > progressively (without intermediate copy), according to its own
> > > MTU
> > > limitation?
> > > 
> > 
> > t7xx driver needs to add metadata to the SKB head for each
> > fragment, so
> > the driver has to allocate a new buffer to copy data(skb_put_data)
> > and
> > insert metadata.
> 
> Normally, once the first part (chunk) of the skb has been consumed
> (skb_pull) and written to the device, it will become part of the
> skb headroom, which can then be used for appending (skb_push) the
> header (metadata) of the second chunks, and so... right?
> 
> Just want to avoid a bunch of unnecessary copy/alloc here.
> 
t7xx DMA can transfer multiple fragments at once, if done as
recomended, the DMA performance will be inhibited.
> Regards,
> Loic