netdev - 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: Mon, 28 Aug 2023 08:38:10 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Alexander Duyck <alexander.duyck@...il.com>
Cc: Yunsheng Lin <linyunsheng@...wei.com>, 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 Mon, 28 Aug 2023 07:50:33 -0700 Alexander Duyck wrote:
> Actually we could keep it pretty simple. We just have to create a
> #define using DMA_BIT_MASK for the size of the page pool DMA. We could
> name it something like PP_DMA_BIT_MASK. The drivers would just have to
> use that to define their bit mask when they call
> dma_set_mask_and_coherent. In that case the DMA API would switch to
> bounce buffers automatically in cases where the page DMA address would
> be out of bounds.
> 
> The other tweak we could look at doing would be to just look at the
> dma_get_required_mask and add a warning and/or fail to load page pool
> on systems where the page pool would not be able to process that when
> ANDed with the device dma mask.
> 
> With those two changes the setup should be rock solid in terms of any
> risks of the DMA address being out of bounds, and with minimal
> performance impact as we would have verified all possibilities before
> we even get into the hot path.

Sounds like a plan!