[PATCH v3 3/4] iommu/arm-smmu: Add support to handle Qcom's wait-for-safe logic

From: Vivek Gautam
Date: Wed Jun 12 2019 - 03:21:06 EST

Qcom's implementation of arm,mmu-500 adds a WAIT-FOR-SAFE logic
to address under-performance issues in real-time clients, such as
Display, and Camera.
On receiving an invalidation requests, the SMMU forwards SAFE request
to these clients and waits for SAFE ack signal from real-time clients.
The SAFE signal from such clients is used to qualify the start of
This logic is controlled by chicken bits, one for each - MDP (display),
IFE0, and IFE1 (camera), that can be accessed only from secure software
on sdm845.

This configuration, however, degrades the performance of non-real time
clients, such as USB, and UFS etc. This happens because, with wait-for-safe
logic enabled the hardware tries to throttle non-real time clients while
waiting for SAFE ack signals from real-time clients.

On MTP sdm845 devices, with wait-for-safe logic enabled at the boot time
by the bootloaders we see degraded performance of USB and UFS when kernel
enables the smmu stage-1 translations for these clients.
Turn off this wait-for-safe logic from the kernel gets us back the perf
of USB and UFS devices until we re-visit this when we start seeing perf
issues on display/camera on upstream supported SDM845 platforms.

Now, different bootloaders with their access control policies allow this
register access differently through secure monitor calls -
1) With one we can issue io-read/write secure monitor call (qcom-scm)
to update the register, while,
2) With other, such as one on MTP sdm845 we should use the specific
qcom-scm command to send request to do the complete register
Adding a separate device tree flag for arm-smmu to identify which
firmware configuration of the two mentioned above we use.
Not adding code change to allow type-(1) bootloaders to toggle the
safe using io-read/write qcom-scm call.

This change is inspired by the downstream change from Patrick Daly
to address performance issues with display and camera by handling
this wait-for-safe within separte io-pagetable ops to do TLB
maintenance. So a big thanks to him for the change.

Without this change the UFS reads are pretty slow:
$ time dd if=/dev/sda of=/dev/zero bs=1048576 count=10 conv=sync
10+0 records in
10+0 records out
10485760 bytes (10.0MB) copied, 22.394903 seconds, 457.2KB/s
real 0m 22.39s
user 0m 0.00s
sys 0m 0.01s

With this change they are back to rock!
$ time dd if=/dev/sda of=/dev/zero bs=1048576 count=300 conv=sync
300+0 records in
300+0 records out
314572800 bytes (300.0MB) copied, 1.030541 seconds, 291.1MB/s
real 0m 1.03s
user 0m 0.00s
sys 0m 0.54s

Signed-off-by: Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx>
drivers/iommu/arm-smmu.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
index 0ad086da399c..3c3ad43eda97 100644
--- a/drivers/iommu/arm-smmu.c
+++ b/drivers/iommu/arm-smmu.c
@@ -39,6 +39,7 @@
#include <linux/pci.h>
#include <linux/platform_device.h>
#include <linux/pm_runtime.h>
+#include <linux/qcom_scm.h>
#include <linux/slab.h>
#include <linux/spinlock.h>

@@ -177,6 +178,7 @@ struct arm_smmu_device {
u32 features;

u32 options;
enum arm_smmu_arch_version version;
enum arm_smmu_implementation model;
@@ -262,6 +264,7 @@ static bool using_legacy_binding, using_generic_binding;

static struct arm_smmu_option_prop arm_smmu_options[] = {
{ ARM_SMMU_OPT_SECURE_CFG_ACCESS, "calxeda,smmu-secure-config-access" },
+ { ARM_SMMU_OPT_QCOM_FW_IMPL_SAFE_ERRATA, "qcom,smmu-500-fw-impl-safe-errata" },
{ 0, NULL},

@@ -2292,6 +2295,19 @@ static int arm_smmu_device_probe(struct platform_device *pdev)

+ /*
+ * To address performance degradation in non-real time clients,
+ * such as USB and UFS, turn off wait-for-safe on sdm845 platforms,
+ * such as MTP, whose firmwares implement corresponding secure monitor
+ * call handlers.
+ */
+ if (of_device_is_compatible(smmu->dev->of_node, "qcom,sdm845-smmu-500") &&
+ err = qcom_scm_qsmmu500_wait_safe_toggle(0);
+ if (err)
+ dev_warn(dev, "Failed to turn off SAFE logic\n");
+ }
* We want to avoid touching dev->power.lock in fastpaths unless
* it's really going to do something useful - pm_runtime_enabled()
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
of Code Aurora Forum, hosted by The Linux Foundation