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Date: Mon, 31 Jul 2023 19:47:08 +0200
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From: Jesper Dangaard Brouer <hawk@kernel.org>
Subject: Re: [PATCH net-next 3/3] bnxt_en: Let the page pool manage the DMA
 mapping
To: Jakub Kicinski <kuba@kernel.org>, Michael Chan <michael.chan@broadcom.com>
Cc: davem@davemloft.net, netdev@vger.kernel.org, edumazet@google.com,
 pabeni@redhat.com, gospo@broadcom.com, bpf@vger.kernel.org,
 somnath.kotur@broadcom.com, Jesper Dangaard Brouer <hawk@kernel.org>,
 Ilias Apalodimas <ilias.apalodimas@linaro.org>
References: <20230728231829.235716-1-michael.chan@broadcom.com>
 <20230728231829.235716-4-michael.chan@broadcom.com>
 <20230728174212.64000bdc@kernel.org>
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On 29/07/2023 02.42, Jakub Kicinski wrote:
> On Fri, 28 Jul 2023 16:18:29 -0700 Michael Chan wrote:
>> +	pp.dma_dir = bp->rx_dir;
>> +	pp.max_len = BNXT_RX_PAGE_SIZE;
> 
> I _think_ you need PAGE_SIZE here.
> 

I actually think pp.max_len = BNXT_RX_PAGE_SIZE is correct here.
(Although it can be optimized, see below)

> 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.


> Adding Jesper to CC to keep me honest.

Adding Ilias to keep me honest ;-)

To follow/understand these changes, reviewers need to keep the context
of patch 1/3 in mind [1].

[1] 
https://lore.kernel.org/all/20230728231829.235716-2-michael.chan@broadcom.com/


> 
>> +	pp.flags = PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV;
> 

--Jesper