Hi Laurent and Suman,
On Fri, Mar 14, 2014 at 12:00:16PM +0100, Laurent Pinchart wrote:
Hi Suman,
(CC'ing Joerg Roedel and Marek Szyprowski for the core IOMMU discussion)
On Thursday 13 March 2014 21:33:37 Suman Anna wrote:
On 03/07/2014 06:46 PM, Laurent Pinchart wrote:
Hello,
This patch set fixes miscellaneous issues with the OMAP IOMMU driver,
found when trying to port the OMAP3 ISP away from omap-iovmm to the ARM
DMA API. The biggest issue is fixed by patch 5/5, while the other patches
fix smaller problems that I've noticed when reading the code, without
experiencing them at runtime.
I'd like to take this as an opportunity to discuss OMAP IOMMU integration
with the ARM DMA mapping implementation. The idea is to hide the IOMMU
completely behind the DMA mapping API, making it completely transparent
to drivers.
Thanks for starting the discussion.
A drivers will only need to allocate memory with dma_alloc_*, and behind
the scene the ARM DMA mapping implementation will find out that the
device is behind an IOMMU and will map the buffers through the IOMMU,
returning an I/O VA address to the driver. No direct call to the OMAP
IOMMU driver or to the IOMMU core should be necessary anymore.
To use the IOMMU the ARM DMA implementation requires a VA mapping to be
created with a call to arm_iommu_create_mapping() and to then be attached
to the device with arm_iommu_attach_device(). This is currently not
performed by the OMAP IOMMU driver, I have thus added that code to the
OMAP3 ISP driver for now. I believe this to still be an improvement
compared to the current situation, as it allows getting rid of custom
memory allocation code in the OMAP3 ISP driver and custom I/O VA space
management in omap-iovmm. However, that code should ideally be removed
from the driver. The question is, where should it be moved to ?
One possible solution would be to add the code to the OMAP IOMMU driver.
However, this would require all OMAP IOMMU users to be converted to the
ARM DMA API. I assume there would be issues that I don't foresee though.
Yeah, I think this will pose some problems for the other main user of IOMMUs
- the remoteproc devices (DSP, Dual-Cortex M3/M4 processors in OMAP4 and
beyond). A remoteproc device also needs to map memory at specific addresses
for its code and data sections, and not rely on a I/O VA address being given
to it. The firmware sections are already linked at specific addresses, and
so remoteproc needs to allocate memory, load the firmware and map into
appropriate device addresses. We are doing this currently usage a
combination of CMA memory to get contiguous memory (there are some
restrictions for certain sections) and iommu_map/unmap API to program the
MMU with these pages. This usage is different from what is expected from
exchanging buffers, which can be allocated from a predefined mapping range.
Even that one is tricky if we need to support different cache
properties/attributes as the cache configuration is in general local to
these processors.
Right, we indeed need two levels of API, one for drivers such as remoteproc
that need direct control of the IOMMU, and one for drivers that only need to
map buffers without any additional requirement. In the second case the
drivers
should ideally use the DMA mapping API not even be aware that an IOMMU is
present. This would require moving the ARM mapping allocation out of the
client driver.
The IOMMU core or the IOMMU driver will need to know whether the driver
expects to control the IOMMU directly or to have it managed transparently. As
this is a software configuration I don't think the information belongs to DT.
The question is, how should this information be conveyed ?
The driver would need to know that, I think.
Currently the DMA mapping API doesn't allow explicit addressing to IOVA
address space AFAIK. The IOMMU API does. It's a good question how to do this
as I don't think there's even a way for the driver to explicitly obtain
access to the IOMMU.
The virtual address space allocation would need to take into account that
some of the address space is actually mapped outside it. The iova library
can do this already.
I'm not even sure whether the OMAP IOMMU driver would be the best place to
put that code. Ideally VA spaces should be created by the platform
somehow, and mapping of devices to IOMMUs should be handled by the IOMMU
core instead of the IOMMU drivers. We're not there yet, and the path
might not be straightforward, hence this attempt to start a constructive
discussion.
A second completely unrelated problem that I'd like to get feedback on is
the suspend/resume support in the OMAP IOMMU driver, or rather the lack
thereof. The driver exports omap_iommu_save_ctx() and
omap_iommu_restore_ctx() functions and expects the IOMMU users to call
