Re: [PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC

From: Robin Murphy
Date: Wed Jul 14 2021 - 06:29:23 EST

On 2021-07-14 11:15, Joerg Roedel wrote:
Hi Robin,

On Fri, Jul 09, 2021 at 02:56:47PM +0100, Robin Murphy wrote:
As I mentioned before, conceptually I think this very much belongs in sysfs
as a user decision. We essentially have 4 levels of "strictness":

1: DMA domain with bounce pages
2: DMA domain
3: DMA domain with flush queue
4: Identity domain

Together with reasonable defaults (influenced by compile-time
options) it seems to be a good thing to configure at runtime via

We already have CONFIG_IOMMU_DEFAULT_PASSTHROUGH, which can probably be
extended to be an option list:

- CONFIG_IOMMU_DEFAULT_PASSTHROUGH: Trusted devices are identity

mapped with strict flush
behavior on unmap

- CONFIG_IOMMU_DEFAULT_DMA_LAZY: Trusted devices are DMA mapped
with flush queues for performance

Indeed, I got focused on the sysfs angle, but rearranging the Kconfig default that way to match makes a lot of sense, and is another thing which should fall out really easily from my domain type rework, so I'll add that to my branch now before I forget again.

Untrusted devices always get into the DMA domain with bounce pages by

The defaults can be changed at runtime via sysfs. We already have basic
support for runtime switching of the default domain, so that can be

As mentioned yesterday, already done! I'm hoping to be able to post the patches next week after some testing :)