Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20221026011540.8499-1-haozhe.chang@mediatek.com>
 <CAMZdPi_XSWeTf-eP+O2ZXGXtn5yviEp=p1Q0rs_fG76UGf2FsQ@mail.gmail.com> <82a7acf3176c90d9bea773bb4ea365745c1a1971.camel@mediatek.com>
In-Reply-To: <82a7acf3176c90d9bea773bb4ea365745c1a1971.camel@mediatek.com>
From:   Loic Poulain <loic.poulain@linaro.org>
Date:   Wed, 26 Oct 2022 16:27:47 +0200
Message-ID: <CAMZdPi_tTBgqSGCUaB29ifOUSE5nWa6ooOa=4k8T6pXJDfpO-A@mail.gmail.com>
Subject: Re: [PATCH] wwan: core: Support slicing in port TX flow of WWAN subsystem
To:     haozhe chang <haozhe.chang@mediatek.com>
Cc:     "chandrashekar.devegowda@intel.com" 
        <chandrashekar.devegowda@intel.com>,
        "linuxwwan@intel.com" <linuxwwan@intel.com>,
        "chiranjeevi.rapolu@linux.intel.com" 
        <chiranjeevi.rapolu@linux.intel.com>,
        =?UTF-8?B?SGFpanVuIExpdSAo5YiY5rW35YabKQ==?= 
        <haijun.liu@mediatek.com>,
        "m.chetan.kumar@linux.intel.com" <m.chetan.kumar@linux.intel.com>,
        "ricardo.martinez@linux.intel.com" <ricardo.martinez@linux.intel.com>,
        "ryazanov.s.a@gmail.com" <ryazanov.s.a@gmail.com>,
        "johannes@sipsolutions.net" <johannes@sipsolutions.net>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "edumazet@google.com" <edumazet@google.com>,
        "kuba@kernel.org" <kuba@kernel.org>,
        "pabeni@redhat.com" <pabeni@redhat.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?TGFtYmVydCBXYW5nICjnjovkvJ8p?= 
        <Lambert.Wang@mediatek.com>,
        =?UTF-8?B?WGlheXUgWmhhbmcgKOW8oOWkj+Wuhyk=?= 
        <Xiayu.Zhang@mediatek.com>, srv_heupstream@mediatek.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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.

Regards,
Loic