Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

[Yunsheng Lin <linyunsheng@huawei.com>]

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;
> .
>