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

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Message-ID: <CA+SN3soUmtYfM_qVQ7L1gHMSLYe2bDm=6U9UwFLvj35odT0Feg@mail.gmail.com>
Date: Tue, 30 Jul 2024 21:35:08 +0300
From: Elad Yifee <eladwf@...il.com>
To: Joe Damato <jdamato@...tly.com>, Elad Yifee <eladwf@...il.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>
Cc: Stefan Roese <sr@...x.de>
Subject: Re: [PATCH net-next v2 1/2] net: ethernet: mtk_eth_soc: use prefetch methods

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)