Re: [RFC: DMA_PMD 01/22] iommu/dma: introduce CONFIG_DMA_PMD and metadata table

[Luigi Rizzo]

On Sat, Oct 3, 2026 at 11:44 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
>
>
>
> On 10/3/26 2:22 PM, Luigi Rizzo wrote:
> > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > index 6e07bd69467a3..1bb347fd9da4a 100644
> > --- a/drivers/iommu/Kconfig
> > +++ b/drivers/iommu/Kconfig
> > @@ -158,6 +158,38 @@ config IOMMU_DMA
> >       select NEED_SG_DMA_LENGTH
> >       select NEED_SG_DMA_FLAGS if SWIOTLB
> >
> > +# PMD_SIZE IOMMU backing for DMA-coherent devices
> > +config DMA_PMD
...
> > +       Costs 128KB of metadata per GB of DMA_PMD buffers, allocated on demand.
>
> Paragraph 1 of the patch description says 64KB per 1GB, not 128KB.
> Or am I mixing things up?

256B per 2MB page, 128KB per 1GB is the correct version.
The number in the patch description is a leftover from an earlier
version, I will update.

cheers
luigi