linux-kernel - Re: [PATCH net-next v6 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

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Date:   Wed, 16 Aug 2023 20:49:19 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>
CC:     <davem@...emloft.net>, <kuba@...nel.org>, <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>,
        <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH net-next v6 1/6] page_pool: frag API support for 32-bit
 arch with 64-bit DMA

On 2023/8/16 19:26, Ilias Apalodimas wrote:
> Hi Yunsheng
> 
> On Mon, 14 Aug 2023 at 15:59, Yunsheng Lin <linyunsheng@...wei.com> wrote:
>>
>> Currently page_pool_alloc_frag() is not supported in 32-bit
>> arch with 64-bit DMA because of the overlap issue between
>> pp_frag_count and dma_addr_upper in 'struct page' for those
>> arches, which seems to be quite common, see [1], which means
>> driver may need to handle it when using frag API.
> 
> That wasn't so common. IIRC it was a single TI platform that was breaking?

I am not so sure about that as grepping 'ARM_LPAE' has a long
list for that.

> 
>>
>> In order to simplify the driver's work when using frag API
>> this patch allows page_pool_alloc_frag() to call
>> page_pool_alloc_pages() to return pages for those arches.
> 
> Do we have any use cases of people needing this?  Those architectures
> should be long dead and although we have to support them in the
> kernel,  I don't personally see the advantage of adjusting the API to
> do that.  Right now we have a very clear separation between allocating
> pages or fragments.   Why should we hide a page allocation under a
> frag allocation?  A driver writer can simply allocate pages for those
> boards.  Am I the only one not seeing a clean win here?

It is also a part of removing the per page_pool PP_FLAG_PAGE_FRAG flag
in this patchset.

> 
> Thanks
> /Ilias
>