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

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Date: Mon, 31 Jul 2023 13:20:04 -0700
From: Michael Chan <michael.chan@...adcom.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>, davem@...emloft.net, netdev@...r.kernel.org, 
	edumazet@...gle.com, pabeni@...hat.com, gospo@...adcom.com, 
	bpf@...r.kernel.org, somnath.kotur@...adcom.com, 
	Ilias Apalodimas <ilias.apalodimas@...aro.org>
Subject: Re: [PATCH net-next 3/3] bnxt_en: Let the page pool manage the DMA mapping

On Mon, Jul 31, 2023 at 11:44 AM Jakub Kicinski <kuba@...nel.org> wrote:
> Maybe I'm misunderstanding. Let me tell you how I think this works and
> perhaps we should update the docs based on this discussion.
>
> Note that the max_len is applied to the full host page when the full
> host page is returned. Not to fragments, and not at allocation.
>

I think I am beginning to understand what the confusion is.  These 32K
page fragments within the page may not belong to the same (GRO)
packet.  So we cannot dma_sync the whole page at the same time.
Without setting PP_FLAG_DMA_SYNC_DEV, the driver code should be
something like this:

mapping = page_pool_get_dma_addr(page) + offset;
dma_sync_single_for_device(dev, mapping, BNXT_RX_PAGE_SIZE, bp->rx_dir);

offset may be 0, 32K, etc.

Since the PP_FLAG_DMA_SYNC_DEV logic is not aware of this offset, we
actually must do our own dma_sync and not use PP_FLAG_DMA_SYNC_DEV in
this case.  Does that sound right?

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