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Subject: Re: [PATCH] wwan: core: Support slicing in port TX flow of WWAN
 subsystem
From:   haozhe chang <haozhe.chang@mediatek.com>
To:     Loic Poulain <loic.poulain@linaro.org>
CC:     "chandrashekar.devegowda@intel.com" 
        <chandrashekar.devegowda@intel.com>,
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        Haijun Liu =?UTF-8?Q?=28=E5=88=98=E6=B5=B7=E5=86=9B=29?= 
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        Lambert Wang =?UTF-8?Q?=28=E7=8E=8B=E4=BC=9F=29?= 
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Date:   Thu, 27 Oct 2022 09:19:42 +0800
In-Reply-To: <CAMZdPi_tTBgqSGCUaB29ifOUSE5nWa6ooOa=4k8T6pXJDfpO-A@mail.gmail.com>
References: <20221026011540.8499-1-haozhe.chang@mediatek.com>
         <CAMZdPi_XSWeTf-eP+O2ZXGXtn5yviEp=p1Q0rs_fG76UGf2FsQ@mail.gmail.com>
         <82a7acf3176c90d9bea773bb4ea365745c1a1971.camel@mediatek.com>
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On Wed, 2022-10-26 at 22:27 +0800, Loic Poulain wrote:
> On Wed, 26 Oct 2022 at 13:45, haozhe chang <haozhe.chang@mediatek.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@mediatek.com> wrote:
> > > > 
> > > > From: haozhe chang <haozhe.chang@mediatek.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