[PATCH v2 2/3] iommu/dma: Use fast DMA domain lookup

 + dri-devel

This patch may also break other ARM DRM drivers. I cced dri-devel so
that they could manage this.

And below relevant link for ARM DRM maintainers,
https://www.spinics.net/lists/arm-kernel/msg676098.html

Thanks,
Inki Dae

2018? 9? 29? (?) ?? 1:19, Marek Szyprowski [off-list ref]?? ??:

Hi Robin,

On 2018-09-28 17:31, Robin Murphy wrote:

quoted

On 28/09/18 15:21, Marek Szyprowski wrote:

quoted

On 2018-09-28 15:52, Robin Murphy wrote:

quoted

On 28/09/18 14:26, Marek Szyprowski wrote:

quoted

On 2018-09-12 17:24, Robin Murphy wrote:

quoted

Most parts of iommu-dma already assume they are operating on a
default
domain set up by iommu_dma_init_domain(), and can be converted
straight
over to avoid the refcounting bottleneck. MSI page mappings may be in
an unmanaged domain with an explicit MSI-only cookie, so retain the
non-specific lookup, but that's OK since they're far from a contended
fast path either way.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>

This breaks Exynos DRM driver with Exynos IOMMU on ARM64 (Exynos5433).
Exynos DRM creates it's own domain, attach all devices which performs
DMA access (typically CRTC devices) to it and uses standard
DMA-mapping
calls to allocate/map buffers. This way it can use the same one DMA
address for each buffer regardless of the CRTC/display/processing
device.

This no longer works with this patch. The simplest solution to fix
this
is add API to change default_domain to the one allocated by the Exynos
DRM driver.

Ugh, I hadn't realised there were any drivers trying to fake up their
own default domains - that's always going to be fragile. In fact, one
of my proposals for un-breaking Tegra and other DRM drivers involves
preventing iommu-dma from trying to operate on non-IOMMU_DOMAIN_DMA
domains at all, which would stop that from working permanently.

IMHO there should be a way to let drivers like DRM to use DMA-mapping
infrastructure on their own iommu domain. Better provide such API to
avoid hacking in the DRM drivers to get this done without help from
IOMMU core.

The tricky part is how to reconcile that with those other drivers
which want to do explicit IOMMU management with their own domain but
still use the DMA API for coherency of the underlying memory. I do
have a couple of half-formed ideas, but they're not quite there yet.

The only idea I have here is to add some flags to struct driver to let
device core and frameworks note that this driver wants to manage
everything on their own. Then such drivers, once they settle their IOMMU
domain, should call some simple API to bless it for DMA API.

quoted

quoted

quoted

Can Exynos not put all the DRM components into a single group, or at
least just pick one of the real default domains to attach everyone to
instead of allocating a fake one?

Exynos DRM components are not in the single group. Currently
iommu-groups have no meaning on Exynos5433 and Exynos IOMMU simply
allocate one group per each iommu-master device in the system.

Yeah, a group spanning multiple IOMMU devices would have been a bit of
a hack for your topology, but still *technically* possible ;)

The question if I want to put all DRM components into single IOMMU
group, how to propagate such knowledge from Exynos DRM driver to Exynos
IOMMU driver (groups are created by IOMMU driver)? Anyway, I don't see
any benefits from using IOMMU groups as for now, especially when each
Exynos DRM component has its own, separate IOMMU (or even more than one,
but that a different story).

quoted

quoted

Exynos DRM already selects one of its component devices as the 'main DMA
device'. It is being used for managing buffers if no IOMMU is available,
so it can also use its iommu domain instead of allocating a fake one.
I've checked and it fixes Exynos DRM now. The question is how to merge
this fix to keep bisectability.

From: Marek Szyprowski <m.szyprowski@samsung.com>
Date: Fri, 28 Sep 2018 16:17:27 +0200
Subject: [PATCH] drm/exynos: Use selected dma_dev default iommu domain
instead
   of a fake one

Instead of allocating a fake IOMMU domain for all Exynos DRM components,
simply reuse the default IOMMU domain of the already selected DMA
device.
This allows some design changes in IOMMU framework without breaking
IOMMU
support in Exynos DRM.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
   drivers/gpu/drm/exynos/exynos_drm_iommu.h | 34
++++-------------------
   1 file changed, 6 insertions(+), 28 deletions(-)

diff --git a/drivers/gpu/drm/exynos/exynos_drm_iommu.h

b/drivers/gpu/drm/exynos/exynos_drm_iommu.h
index 87f6b5672e11..51d3b7dcd529 100644

--- a/drivers/gpu/drm/exynos/exynos_drm_iommu.h
+++ b/drivers/gpu/drm/exynos/exynos_drm_iommu.h

@@ -55,37 +55,12 @@ static inline void __exynos_iommu_detach(struct

exynos_drm_private *priv,
   static inline int __exynos_iommu_create_mapping(struct
exynos_drm_private *priv,
                       unsigned long start, unsigned long size)
   {
-    struct iommu_domain *domain;
-    int ret;
-
-    domain = iommu_domain_alloc(priv->dma_dev->bus);
-    if (!domain)
-        return -ENOMEM;
-
-    ret = iommu_get_dma_cookie(domain);
-    if (ret)
-        goto free_domain;
-
-    ret = iommu_dma_init_domain(domain, start, size, NULL);
-    if (ret)
-        goto put_cookie;
-
-    priv->mapping = domain;
+    priv->mapping = iommu_get_dma_domain(priv->dma_dev);

iommu_get_domain_for_dev(), please - this isn't a performance-critical
path inside a DMA API implementation, plus without an actual
dependency on this series maybe there's a chance of sneaking it into
4.19?

(you could always still check domain->type == IOMMU_DOMAIN_DMA if you
want to be really really paranoid.)

I've sent a patch. I hope we will manage to get it as a fix into v4.19,
so it won't block your changes in v4.20.

quoted

quoted

       return 0;
-
-put_cookie:
-    iommu_put_dma_cookie(domain);
-free_domain:
-    iommu_domain_free(domain);
-    return ret;
   }

   static inline void __exynos_iommu_release_mapping(struct
exynos_drm_private *priv)
   {
-    struct iommu_domain *domain = priv->mapping;
-
-    iommu_put_dma_cookie(domain);
-    iommu_domain_free(domain);
       priv->mapping = NULL;
   }

@@ -94,7 +69,9 @@ static inline int __exynos_iommu_attach(struct

exynos_drm_private *priv,
   {
       struct iommu_domain *domain = priv->mapping;

-    return iommu_attach_device(domain, dev);
+    if (dev != priv->dma_dev)
+        return iommu_attach_device(domain, dev);
+    return 0;
   }

   static inline void __exynos_iommu_detach(struct exynos_drm_private
*priv,

@@ -102,7 +79,8 @@ static inline void __exynos_iommu_detach(struct

exynos_drm_private *priv,
   {
       struct iommu_domain *domain = priv->mapping;

-    iommu_detach_device(domain, dev);
+    if (dev != priv->dma_dev)
+        iommu_detach_device(domain, dev);

Strictly you could skip that check, as detaching the master device
from its real default domain is just a no-op, but I guess maintaining
the symmetry is probably more intuitive.

I would like to keep the symmetry.

quoted

...

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland