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Message-ID: <5bd4ba5d-c364-f3f6-bbeb-903d71102ea2@huawei.com>
Date: Tue, 22 Aug 2023 17:21:35 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Jakub Kicinski <kuba@...nel.org>
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>,
Alexander Duyck <alexander.duyck@...il.com>,
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 2023/8/22 2:35, Jakub Kicinski wrote:
> On Mon, 21 Aug 2023 20:18:55 +0800 Yunsheng Lin wrote:
>>> - page_pool_set_dma_addr(page, dma);
>>> + if (page_pool_set_dma_addr(page, dma))
>>> + goto unmap_failed;
>>
>> What does the driver do when the above fails?
>> Does the driver still need to implement a fallback for 32 bit arch with
>> dma addr with more than 32 + 12 bits?
>> If yes, it does not seems to be very helpful from driver's point of view
>> as the driver might still need to call page allocator API directly when
>> the above fails.
>
> I'd expect the driver to do nothing, we are operating under
> the assumption that "this will never happen". If it does
> the user should report it back to us. So maybe..
Digging a litte deeper:
>From CPU's point of views, up to 40 bits is supported for LPAE.
>From device's point of view, it seems up to 48-bits is supported
when iommu is enabled, see below:
https://elixir.free-electrons.com/linux/v6.5-rc7/source/drivers/iommu/Kconfig#L28
>
>>> if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
>>> page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
>>>
>>> return true;
>>> +
>>> +unmap_failed:
>
> .. 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
>
> here?
>
>>> + dma_unmap_page_attrs(pool->p.dev, dma,
>>> + PAGE_SIZE << pool->p.order, pool->p.dma_dir,
>>> + DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
>>> + return false;
> .
>
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