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Date: Mon, 31 Jul 2023 11:00:08 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Jesper Dangaard Brouer <hawk@kernel.org>
Cc: Michael Chan <michael.chan@broadcom.com>, davem@davemloft.net,
 netdev@vger.kernel.org, edumazet@google.com, pabeni@redhat.com,
 gospo@broadcom.com, bpf@vger.kernel.org, somnath.kotur@broadcom.com, Ilias
 Apalodimas <ilias.apalodimas@linaro.org>
Subject: Re: [PATCH net-next 3/3] bnxt_en: Let the page pool manage the DMA
 mapping
Message-ID: <20230731110008.26e8ce03@kernel.org>
In-Reply-To: <2eadb48b-2991-7458-16a6-51082ff3ec2c@kernel.org>
References: <20230728231829.235716-1-michael.chan@broadcom.com>
	<20230728231829.235716-4-michael.chan@broadcom.com>
	<20230728174212.64000bdc@kernel.org>
	<2eadb48b-2991-7458-16a6-51082ff3ec2c@kernel.org>
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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.