linux-kernel - Re: [PATCH net-next v3 1/4] page_pool: frag API support for 32-bit arch with 64-bit DMA

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Message-ID: <2c48652e-ace4-45c8-7a7d-5ec87d1b0b75@redhat.com>
Date:   Sun, 11 Jun 2023 12:47:42 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Yunsheng Lin <yunshenglin0825@...il.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>,
        Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
        kuba@...nel.org, pabeni@...hat.com
Cc:     brouer@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH net-next v3 1/4] page_pool: frag API support for 32-bit
 arch with 64-bit DMA


On 10/06/2023 15.13, Yunsheng Lin wrote:
> On 2023/6/9 23:02, Jesper Dangaard Brouer wrote:
> ...
> 
>>>                     PP_FLAG_DMA_SYNC_DEV |\
>>>                     PP_FLAG_PAGE_FRAG)
>>>    +#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT    \
>>> +        (sizeof(dma_addr_t) > sizeof(unsigned long))
>>> +
>>
>> I have a problem with the name PAGE_POOL_DMA_USE_PP_FRAG_COUNT
>> because it is confusing to read in an if-statement.
> 
> Actually, it is already in an if-statement before this patch:)

I did notice, but I've had a problem with this name for a while.
(see later, why this might be long in separate patch)

> Maybe starting to use it in the driver is confusing to you?
> If not, maybe we can keep it that for now, and change it when
> we come up with a better name.
> 
>>
>> Proposals rename to:  DMA_OVERLAP_PP_FRAG_COUNT
>>   Or:  MM_DMA_OVERLAP_PP_FRAG_COUNT
>>   Or:  DMA_ADDR_OVERLAP_PP_FRAG_COUNT
> 
> It seems DMA_ADDR_OVERLAP_PP_FRAG_COUNT is better,
> and DMA_ADDR_UPPER_OVERLAP_PP_FRAG_COUNT seems more accurate if a
> longer macro name is not an issue here.
> 

I like the shorter DMA_ADDR_OVERLAP_PP_FRAG_COUNT variant best.

>>
>> Notice how I also removed the prefix PAGE_POOL_ because this is a
>> MM-layer constraint and not a property of page_pool.
> 
> I am not sure if it is a MM-layer constraint yet.
> Do you mean 'MM-layer constraint' as 'struct page' not having
> enough space for page pool with 32-bit arch with 64-bit DMA?

Yes.

> If that is the case, we may need a more generic name for that
> constraint instead of 'DMA_ADDR_OVERLAP_PP_FRAG_COUNT'?
>

I think this name is clear enough; the dma_addr_t is overlapping the 
pp_frag_count.


> And a more generic name seems confusing for page pool too, as
> it doesn't tell that we only have that problem for 32-bit arch
> with 64-bit DMA.
> 
> So if the above makes sense, it seems we may need to keep the
> PAGE_POOL_ prefix, which would be
> 'PAGE_POOL_DMA_ADDR_UPPER_OVERLAP_PP_FRAG_COUNT' if the long
> name is not issue here.
> 

I think it gets too long now.

Also I still disagree with PAGE_POOL_ prefix, if anything it is a
property of 'struct page'.  Thus a prefix with PAGE_ make more sense to
me, but it also gets too long (for my taste).

> Anyway, naming is hard, we may need a seperate patch to explain
> it, which is not really related to this patchset IHMO, so I'd
> rather keep it as before if we can not come up with a name which
> is not confusing to most people.
> 

Okay, lets do the (re)naming in another patch then.

--Jesper