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

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Date: Tue, 1 Aug 2023 19:06:04 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Michael Chan <michael.chan@...adcom.com>, Jakub Kicinski <kuba@...nel.org>
Cc: 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 31/07/2023 23.11, Michael Chan wrote:
> On Mon, Jul 31, 2023 at 1:44 PM Jakub Kicinski <kuba@...nel.org> wrote:
>> tl;dr just set .max_len = PAGE_SIZE and all will be right.
> 
> OK I think I got it now.  The page is only recycled when all the
> fragments are recycled and so we can let page pool DMA sync the whole
> page at that time.

Yes, Jakub is right, I see that now.

When using page_pool "frag" API (e.g. page_pool_dev_alloc_frag) then the
optimization I talked about isn't valid.  We simply have to DMA sync the
entire page, when it gets back to the recycle stage.

--Jesper