Re: [RFC: DMA_PMD 00/22] DMA_PMD: PMD_SIZE-backed IO buffer pools

[David Hildenbrand (Arm)]

On 10/12/26 16:45, Luigi Rizzo wrote:
> On Mon, Oct 12, 2026 at 3:51 PM David Hildenbrand (Arm)
> <david@xxxxxxxxxx> wrote:
>>
>> On 10/3/26 23:22, Luigi Rizzo wrote:
>>> Here is a subsystem called DMA_PMD on which I would like feedback on
>>> architecture, possible enhancements, or kernel components that could be
>>> reused to avoid duplication.
>>
>> Here is a feedback to a subsystem called DMA_PMD ;)
>>
>> Why is basing this on PMDs (and embedding it in the name) the right approach?
>>
>> On architctures with large PMDs (512 MiB on arm64 with 64k), would we really
>> want to use 512 MiB pages, or rather something smaller (e.g., 2 MiB) that might
>> possibly also have TLB benefits (cont-pte)?
> 
> I had to call it something... an earlier prototype used the name IOMMU_2M,
> but I didn't want to suggest that the design was limited to 2M (it is not).
> 
> Aside from the name, the _current_ prototype restricts to 2M allocations,
> exactly because too large units are expensive and wasteful.
> 
> In the code, decoupling the allocation size from the PMD size is trivial.

Yeah, we should do that.

We should probably just find a way to describe that this is a POOL that focuses
on physical contiguity in some configured granularity.

-- 
Cheers,

David