is dma_mapping_error() check necessary for dma_alloc_noncoherent()?

[Baochen Qiang]

On 8/27/2025 3:31 PM, Christoph Hellwig wrote:
> On Tue, Aug 26, 2025 at 05:44:42PM +0800, Baochen Qiang wrote:
>> Checking code gives me the impression that I should do dma_mapping_error() check as well.
>> And indeed with below diff the warning is gone:
>>
>> +       dma_mapping_error(ab->dev, paddr);
>>
>> However this does not make sense to me since IMO testing the CPU address is good enough, I
>> can not imagine a valid case where DMA alloc/map fails while returning a valid CPU
>> address, no?
> 
> Yes, this doesn't make sense.  dma_mapping_error exists to provide a
> error handling path for dma_map_*, which return the dma address only.
> 
> For the dma_alloc_* interfaces that return a pointer and can signal
> with a NULL return it should not be needed and dma-debug needs to
> be fixed.
> 
>> If I was right, should we remove invocation to debug_dma_map_page() in dma_alloc_pages()?
>>
> 
> That allocation still needs to be tracked, so it can't just be removed
> but needs to be changed to record the kind of allocation.

Thanks, I will submit a patch to fix this.