Re: [PATCH rfc v5 2/4] page_pool: add interface to manipulate frag count in page pool

[Yunsheng Lin]

On 2021/7/14 22:46, Alexander Duyck wrote:
> On Wed, Jul 14, 2021 at 3:18 AM Jesper Dangaard Brouer
> <jbrouer@xxxxxxxxxx> wrote:
>>
>>
>>
>> On 14/07/2021 11.34, Yunsheng Lin wrote:
>>> As suggested by Alexander, "A DMA mapping should be page
>>> aligned anyway so the lower 12 bits would be reserved 0",
>>> so it might make more sense to repurpose the lower 12 bits
>>> of the dma address to store the frag count for frag page
>>> support in page pool for 32 bit systems with 64 bit dma,
>>> which should be rare those days.
>>
>> Do we have any real driver users with 32-bit arch and 64-bit DMA, that
>> want to use this new frag-count system you are adding to page_pool?
>>
>> This "lower 12-bit use" complicates the code we need to maintain
>> forever. My guess is that it is never used, but we need to update and
>> maintain it, and it will never be tested.
>>
>> Why don't you simply reject using page_pool flag PP_FLAG_PAGE_FRAG
>> during setup of the page_pool for this case?
>>
>>   if ((pool->p.flags & PP_FLAG_PAGE_FRAG) &&
>>       (sizeof(dma_addr_t) > sizeof(unsigned long)))
>>     goto reject-setup;
>>
>>
> 
> That sounds good to me if we want to go that route. It would simplify
> this quite a bit since essentially we could just drop these if blocks.

Ok, let's wait for a few day to see if there is anyone with 32-bit arch
and 64-bit DMA system care enough to use the frag-count support in the page
pool.

> 
> Thanks.
> 
> - Alex
> .
>