netdev - Re: [PATCH net-next v2 1/2] net: ethernet: mtk_eth

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Message-ID: <17deb48c-6148-4e3d-aa0b-6c840f55302d@denx.de>
Date: Thu, 1 Aug 2024 09:09:27 +0200
From: Stefan Roese <sr@...x.de>
To: Elad Yifee <eladwf@...il.com>, Joe Damato <jdamato@...tly.com>,
 Felix Fietkau <nbd@....name>, Sean Wang <sean.wang@...iatek.com>,
 Mark Lee <Mark-MC.Lee@...iatek.com>, Lorenzo Bianconi <lorenzo@...nel.org>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Matthias Brugger <matthias.bgg@...il.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org,
 Daniel Golle <daniel@...rotopia.org>
Subject: Re: [PATCH net-next v2 1/2] net: ethernet: mtk_eth_soc: use prefetch
 methods

On 7/30/24 20:35, Elad Yifee wrote:
> On Tue, Jul 30, 2024 at 11:59 AM Joe Damato <jdamato@...tly.com> wrote:
>>
>> Based on the code in mtk_probe, I am guessing that only
>> MTK_SOC_MT7628 can DMA to unaligned addresses, because for
>> everything else eth->ip_align would be 0.
>>
>> Is that right?
>>
>> I am asking because the documentation in
>> Documentation/core-api/unaligned-memory-access.rst refers to the
>> case you mention, NET_IP_ALIGN = 0, suggesting that this is
>> intentional for performance reasons on powerpc:
>>
>>    One notable exception here is powerpc which defines NET_IP_ALIGN to
>>    0 because DMA to unaligned addresses can be very expensive and dwarf
>>    the cost of unaligned loads.
>>
>> It goes on to explain that some devices cannot DMA to unaligned
>> addresses and I assume that for your driver that is everything which
>> is not MTK_SOC_MT7628 ?
> 
> I have no explanation for this partial use of 'eth->ip_align', it
> could be a mistake
> or maybe I'm missing something.
> Perhaps Stefan Roese, who wrote this part, has an explanation.
> (adding Stefan to CC)

Sorry, I can't answer this w/o digging deeper into this driver and
SoC again. And I didn't use it for a few years now. It might be a
mistake.

Thanks,
Stefan