Re: [PATCH 0/3] uio/dma-buf: Give UIO users access to DMA addresses.

[Christoph Hellwig <hch@infradead.org>]

On Mon, Apr 14, 2025 at 10:24:55AM +0200, Thomas Petazzoni wrote:
> What this patch series is about is to add new user-space interface to
> extend the existing UIO subsystem.

Which as I explained to you is fundamentally broken and unsafe.  If you
need to do DMA from userspae you need to use vfio/iommufd.

> I am not sure how this can work in our use-case. We have a very simple
> set of IP blocks implemented in a FPGA, some of those IP blocks are
> able to perform DMA operations. The register of those IP blocks are
> mapped into a user-space application using the existing, accepted
> upstream, UIO subsystem. Some of those registers allow to program DMA
> transfers. So far, we can do all what we need, except program those DMA
> transfers. Lots of people are having the same issue, and zillions of
> ugly out-of-tree solutions flourish all over, and we're trying to see
> if we can constructively find a solution that would be acceptable
> upstream to resolve this use-case. Our platform is an old PowerPC with
> no IOMMU.

Then your driver design can't work and you need to replace it with a
proper in-kernel driver.