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From:   Loic Poulain <loic.poulain@linaro.org>
Date:   Sat, 29 Oct 2022 10:04:01 +0200
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Subject: Re: [PATCH] wwan: core: Support slicing in port TX flow of WWAN subsystem
To:     haozhe chang <haozhe.chang@mediatek.com>
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On Thu, 27 Oct 2022 at 03:19, haozhe chang <haozhe.chang@mediatek.com> wrote:
>
> 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.

OK, so the skb fragmentation is valid in t7xx case, but the way of
doing it is kind of specific to t7xx. Maybe a more acceptable solution
for a generic fragmentation feature would be to keep the extra
fragments part of the 'main' skb, using skb chaining. That would allow
the fragments to stay linked to a specific user transfer. So if
fragmentation is enabled for a given driver, core only fills the
initial skb with MTU size, and appends additional skb as fragments,
you can look at mhi_net_skb_agg() for skb chaining example.

Regards,
Loic