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

[Loic Poulain]

On Wed, 26 Oct 2022 at 13:45, haozhe chang <haozhe.chang@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> wrote:
> > >
> > > From: haozhe chang <haozhe.chang@xxxxxxxxxxxx>
> > >
> > > 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