[PATCH v2] iommu: add support for drivers that manage iommu explicitly

Rob Clark
2019-07-10T18:38:38Z

From: Rob Clark <robdclark@chromium.org>

Avoid attaching any non-driver managed domain if the driver indicates
that it manages the iommu directly.

This avoids a problem on devices where the bootloader takes the SMMU out
of bypass and enables scanout, such as is the case on snapdragon aarch64
laptops and newer snapdragon android devices. Attaching an IDENTITY or
DMA domain before the driver has a chance to intervene will break efifb
scanout and start triggering iommu faults.

If the driver manages the iommu directly (as does drm/msm), it can
shut down scanout when it is ready to take over the display, before
attaching an UNMANAGED domain.

Signed-off-by: Rob Clark <robdclark@chromium.org>
---
v2. Move the check into arm_smmu_attach_dev() (as I *think* this is
what Robin preferred; update commit msg to focus on the display
related issue that this solves.

We also need Bjorn's patch set to inherit SMR and CB config during
init:

https://www.spinics.net/lists/arm-kernel/msg732246.html

drivers/iommu/arm-smmu.c | 11 +++++++++++
include/linux/device.h | 3 ++-
2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
index 1a5efa7c8767..4a80710124db 100644
--- a/drivers/iommu/arm-smmu.c
+++ b/drivers/iommu/arm-smmu.c
@@ -1411,6 +1411,17 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
return -ENXIO;
}

+ /*
+ * If driver is going to manage iommu directly, then avoid
+ * attaching any non driver managed domain. There could
+ * be already active dma underway (ie. scanout in case of
+ * bootloader enabled display), and interfering with that
+ * will make things go *boom*
+ */
+ if ((domain->type != IOMMU_DOMAIN_UNMANAGED) &&
+ dev->driver && dev->driver->driver_manages_iommu)
+ return 0;
+
/*
* FIXME: The arch/arm DMA API code tries to attach devices to its own
* domains between of_xlate() and add_device() - we have no way to cope
diff --git a/include/linux/device.h b/include/linux/device.h
index e138baabe01e..d98aa4d3c8c3 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -282,7 +282,8 @@ struct device_driver {
struct module *owner;
const char *mod_name; /* used for built-in modules */

- bool suppress_bind_attrs; /* disables bind/unbind via sysfs */
+ bool suppress_bind_attrs:1; /* disables bind/unbind via sysfs */
+ bool driver_manages_iommu:1; /* driver manages IOMMU explicitly */
enum probe_type probe_type;

const struct of_device_id *of_match_table;
--
2.20.1