Messages in this thread | | | From | Haozhe Chang (常浩哲) <> | Subject | Re: [PATCH] wwan: core: Support slicing in port TX flow of WWAN subsystem | Date | Wed, 2 Nov 2022 05:27:07 +0000 |
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On Sat, 2022-10-29 at 10:04 +0200, Loic Poulain wrote: > 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. > OK, It is a good suggestion, I will implement it in patch v2. Also, any advice on reserved headroom? > Regards, > Loic
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