Re: [RFC] dma/mapping.c: WARN_ONCE on dma_addressing_limited() being true

[Balbir Singh]

On 4/14/25 15:54, Christoph Hellwig wrote:
> On Sat, Apr 12, 2025 at 07:41:10PM +1000, Balbir Singh wrote:
>> In the debug and resolution of an issue involving forced use of bounce
>> buffers, 7170130e4c72 ("x86/mm/init: Handle the special case of device
>> private pages in add_pages(), to not increase max_pfn and trigger
>> dma_addressing_limited() bounce buffers"). It would have been easier
>> to debug the issue if dma_addressing_limited() had a warning about a
>> device not being able to address all of memory and thus forcing all
>> accesses through a bounce buffer. Please see[2].
>>
>> A warning would have let the user of the system know that in their
>> particular case, use_dma32 is set due to the addressing limitation
>> and this would impact performance of the driver in use.
>>
>> Implement a WARN_ONCE() to point to the potential use of bounce buffers
>> when we hit the condition. When swiotlb is used,
>> dma_addressing_limited() is used to determine the size of maximum dma
>> buffer size in dma_direct_max_mapping_size(). The warning could be
>> triggered in that check as well.
> 
> dma_addressing_limited is a perfectly expected condition, and returns
> true for many devices and still plenty system configuation.  A kernel
> warning with a stacktrace is not acceptable for that.  A simple one-line
> dev_info might be ok, but could still be rather spammy on many systems.
> 

Thanks for the review!

I'll convert it to a dev_info(). We can remove it, if it causes confusion
or users complain about it?

Balbir