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

[Luigi Rizzo]

On Wed, Oct 14, 2026 at 12:48 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 10/13/26 12:42, Robin Murphy wrote:
> > On 12/10/2026 8:43 pm, David Hildenbrand (Arm) wrote:
> >> On 10/12/26 16:45, Luigi Rizzo wrote:
> >>> On Mon, Oct 12, 2026 at 3:51 PM David Hildenbrand (Arm)
> >>> <david@xxxxxxxxxx> wrote:
> >>>
> >>> 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.
> > And since it's intended for DMA we could call it dma_pool and oh look it already
> > exists...
>
> Heh, I did have "DMA_POOL" in mind when writing my reply.

which confirms that naming is one of the hardest problems in CS...
Let's try to figure out a good name before renaming everything.


> > Sure the current implementation is capped at PAGE_SIZE for allocation (and thus
> > mapping too), but I don't see why that couldn't just be made more flexible.
>
> That intuitively sounds like the right direction.

Point taken, I will decouple the allocaiton size from PMD_SIZE this in
the next iteration.

I have also followed up on most of Pranjal's feedback (stricter error checking,
compact IOVA ranges, opt-in per destination on the tx path...) .

I am also looking at reducing some inefficiencies (e.g. pagepool already caches,
and allocations of new pages are relatively rare, so there is not much
of a point
of having a dma_pmd_pool per queue; and on the tx side, something similar to the
percpu page cache to reduce locking costs when a page is allocated on one CPU
and freed on a different one)

cheers
luigi