On Sun, 26 Jan 2025 10:41:23 +0200, Ido Schimmel wrote: > SPH is the only scenario in which the driver uses multiple buffers per > packet? Yes. Jumbo mode may use multiple buffers per packet too, but they are high order pages, just like a single page in a page pool when using a standard MTU. > > pp_params.max_len = dma_conf->dma_buf_sz; > > Are you sure this is correct? Page pool documentation says that "For > pages recycled on the XDP xmit and skb paths the page pool will use > the max_len member of struct page_pool_params to decide how much of > the page needs to be synced (starting at offset)" [1]. Page pool must sync an area of the buffer because both DMA and CPU may touch this area, other areas are CPU exclusive, so no sync for them seems better. > While "no more than dma_conf->dma_buf_sz bytes will be written into a > page buffer", for the head buffer they will be written starting at a > non-zero offset unlike buffers used for the data, no? Correct, they have different offsets. The "SPH feature" splits header into buf->page (non-zero offset) and splits payload into buf->sec_page (zero offset). For buf->page, pp_params.max_len should be the size of L3/L4 header, and with a offset of NET_SKB_PAD. For buf->sec_page, pp_params.max_len should be dma_conf->dma_buf_sz, and with a offset of 0. This is always true: sizeof(L3/L4 header) + NET_SKB_PAD < dma_conf->dma_buf_sz + 0 pp_params.max_len = dma_conf->dma_buf_sz; make things simpler :)
