linux-kernel - Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

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Date:   Tue, 22 Aug 2023 08:38:21 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Mina Almasry <almasrymina@...gle.com>, <davem@...emloft.net>,
        <pabeni@...hat.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Liang Chen <liangchen.linux@...il.com>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit
 arch with 64-bit DMA

On Tue, 22 Aug 2023 17:21:35 +0800 Yunsheng Lin wrote:
> > .. we should also add a:
> > 
> > 	WARN_ONCE(1, "misaligned DMA address, please report to netdev@");  
> 
> As the CONFIG_PHYS_ADDR_T_64BIT seems to used widely in x86/arm/mips/powerpc,
> I am not sure if we can really make the above assumption.
> 
> https://elixir.free-electrons.com/linux/v6.4-rc6/K/ident/CONFIG_PHYS_ADDR_T_64BIT

Huh, it's actually used a lot less than I anticipated!

None of the x86/arm/mips/powerpc systems matter IMHO - the only _real_
risk is something we don't know about returning non-aligned addresses.

Unless we know about specific problems I'd suggest we took the simpler
path rather than complicating the design for systems which may not
exist.

Alex, do you know of any such cases? Some crazy swiotlb setting?
WDYT about this in general?