Re: [PATCH net-next 3/3] bnxt_en: Let the page pool manage the DMA mapping

[Michael Chan <michael.chan@broadcom.com>]

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On Mon, Jul 31, 2023 at 11:00 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Mon, 31 Jul 2023 19:47:08 +0200 Jesper Dangaard Brouer wrote:
> > > This should be smaller than PAGE_SIZE only if you're wasting the rest
> > > of the buffer, e.g. MTU is 3k so you know last 1k will never get used.
> > > PAGE_SIZE is always a multiple of BNXT_RX_PAGE so you waste nothing.
> >
> > Remember pp.max_len is used for dma_sync_for_device.
> > If driver is smart, it can set pp.max_len according to MTU, as the (DMA
> > sync for) device knows hardware will not go beyond this.
> > On Intel "dma_sync_for_device" is a no-op, so most drivers done
> > optimized for this. I remember is had HUGE effects on ARM EspressoBin board.
>
> Note that (AFAIU) there is no MTU here, these are pages for LRO/GRO,
> they will be filled with TCP payload start to end. page_pool_put_page()
> does nothing for non-last frag, so we'll only sync for the last
> (BNXT_RX_PAGE-sized) frag released, and we need to sync the entire
> host page.

Correct, there is no MTU here.  Remember this matters only when
PAGE_SIZE > BNXT_RX_PAGE_SIZE (e.g. 64K PAGE_SIZE and 32K
BNXT_RX_PAGE_SIZE).  I think we want to dma_sync_for_device for 32K in
this case.

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