Re: [PATCH v1 7/9] iommu/tegra: gart: Provide single domain and group for all devices
From: Robin Murphy
Date: Mon May 14 2018 - 14:18:25 EST
On 11/05/18 21:05, Dmitry Osipenko wrote:
On 11.05.2018 15:32, Robin Murphy wrote:
On 08/05/18 19:16, Dmitry Osipenko wrote:
GART aperture is shared by all devices, hence there is a single IOMMU
domain and group shared by these devices. Allocation of a group per
device only wastes resources and allowance of having more than one domain
is simply wrong because IOMMU mappings made by the users of "different"
domains will stomp on each other.
Strictly, that reasoning is a bit backwards - allocating multiple groups is the
conceptually-wrong thing if the GART cannot differentiate between different
devices, whereas having multiple domains *exist* is no real problem, it's merely
that only one can be active at any point in time (which will inherently become
the case once all devices are grouped together).
IIUC, the IOMMU domain represents the address space. There is only one address
space in a case of GART, the GART's aperture. So GART not only isn't
differentiating between different devices, but also between different domains.
Right, but that's the same as many other IOMMUs (exynos, rockchip, mtk,
etc.) - the point is that an IOMMU domain represents *an* address space,
but if nothing is attached to that domain, it's just a set of logical
mappings which doesn't need to be backed by real hardware. It's
specifically *because* these IOMMUs also can't differentiate between
devices that things work out neatly - there's only one group, which can
only be attached to a single domain at once, so there is never a time
when more than one domain needs to be backed by hardware. Think of the
IOMMU+devices as a CPU and the domains as processes ;)
Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx>
---
 drivers/iommu/tegra-gart.c | 107 +++++++++----------------------------
 1 file changed, 24 insertions(+), 83 deletions(-)
diff --git a/drivers/iommu/tegra-gart.c b/drivers/iommu/tegra-gart.c
index 5b2d27620350..ebc105c201bd 100644
--- a/drivers/iommu/tegra-gart.c
+++ b/drivers/iommu/tegra-gart.c
@@ -19,7 +19,6 @@
  #include <linux/io.h>
 #include <linux/iommu.h>
-#include <linux/list.h>
 #include <linux/module.h>
 #include <linux/of_device.h>
 #include <linux/slab.h>
@@ -44,22 +43,17 @@
 #define GART_PAGE_MASK \
ÂÂÂÂÂ (~(GART_PAGE_SIZE - 1) & ~GART_ENTRY_PHYS_ADDR_VALID)
 -struct gart_client {
-ÂÂÂ struct deviceÂÂÂÂÂÂÂ *dev;
-ÂÂÂ struct list_headÂÂÂ list;
-};
-
 struct gart_device {
ÂÂÂÂÂ void __iomemÂÂÂÂÂÂÂ *regs;
ÂÂÂÂÂ u32ÂÂÂÂÂÂÂÂÂÂÂ *savedata;
ÂÂÂÂÂ u32ÂÂÂÂÂÂÂÂÂÂÂ page_count;ÂÂÂ /* total remappable size */
ÂÂÂÂÂ dma_addr_tÂÂÂÂÂÂÂ iovmm_base;ÂÂÂ /* offset to vmm_area */
ÂÂÂÂÂ spinlock_tÂÂÂÂÂÂÂ pte_lock;ÂÂÂ /* for pagetable */
-ÂÂÂ struct list_headÂÂÂ client;
-ÂÂÂ spinlock_tÂÂÂÂÂÂÂ client_lock;ÂÂÂ /* for client list */
ÂÂÂÂÂ struct deviceÂÂÂÂÂÂÂ *dev;
 Â struct iommu_device iommu; /* IOMMU Core handle */
+ÂÂÂ struct iommu_groupÂÂÂ *group;ÂÂÂÂÂÂÂ /* Common IOMMU group */
+ÂÂÂ struct gart_domainÂÂÂ *domain;ÂÂÂ /* Unique IOMMU domain */
 Â struct tegra_mc_gart_handle mc_gart_handle;
 };
@@ -169,81 +163,31 @@ static inline bool gart_iova_range_valid(struct
gart_device *gart,
 static int gart_iommu_attach_dev(struct iommu_domain *domain,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ struct device *dev)
 {
-ÂÂÂ struct gart_domain *gart_domain = to_gart_domain(domain);
-ÂÂÂ struct gart_device *gart = gart_domain->gart;
-ÂÂÂ struct gart_client *client, *c;
-ÂÂÂ int err = 0;
-
-ÂÂÂ client = devm_kzalloc(gart->dev, sizeof(*c), GFP_KERNEL);
-ÂÂÂ if (!client)
-ÂÂÂÂÂÂÂ return -ENOMEM;
-ÂÂÂ client->dev = dev;
-
-ÂÂÂ spin_lock(&gart->client_lock);
-ÂÂÂ list_for_each_entry(c, &gart->client, list) {
-ÂÂÂÂÂÂÂ if (c->dev == dev) {
-ÂÂÂÂÂÂÂÂÂÂÂ dev_err(gart->dev,
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ "%s is already attached\n", dev_name(dev));
-ÂÂÂÂÂÂÂÂÂÂÂ err = -EINVAL;
-ÂÂÂÂÂÂÂÂÂÂÂ goto fail;
-ÂÂÂÂÂÂÂ }
-ÂÂÂ }
-ÂÂÂ list_add(&client->list, &gart->client);
-ÂÂÂ spin_unlock(&gart->client_lock);
-ÂÂÂ dev_dbg(gart->dev, "Attached %s\n", dev_name(dev));
ÂÂÂÂÂ return 0;
-
-fail:
-ÂÂÂ devm_kfree(gart->dev, client);
-ÂÂÂ spin_unlock(&gart->client_lock);
-ÂÂÂ return err;
 }
  static void gart_iommu_detach_dev(struct iommu_domain *domain,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ struct device *dev)
 {
-ÂÂÂ struct gart_domain *gart_domain = to_gart_domain(domain);
-ÂÂÂ struct gart_device *gart = gart_domain->gart;
-ÂÂÂ struct gart_client *c;
-
-ÂÂÂ spin_lock(&gart->client_lock);
-
-ÂÂÂ list_for_each_entry(c, &gart->client, list) {
-ÂÂÂÂÂÂÂ if (c->dev == dev) {
-ÂÂÂÂÂÂÂÂÂÂÂ list_del(&c->list);
-ÂÂÂÂÂÂÂÂÂÂÂ devm_kfree(gart->dev, c);
-ÂÂÂÂÂÂÂÂÂÂÂ dev_dbg(gart->dev, "Detached %s\n", dev_name(dev));
-ÂÂÂÂÂÂÂÂÂÂÂ goto out;
-ÂÂÂÂÂÂÂ }
-ÂÂÂ }
-ÂÂÂ dev_err(gart->dev, "Couldn't find\n");
-out:
-ÂÂÂ spin_unlock(&gart->client_lock);
 }
The .detach_dev callback is optional in the core API now, so you can just remove
the whole thing.
Good catch, thanks!
 static struct iommu_domain *gart_iommu_domain_alloc(unsigned type)
 {
-ÂÂÂ struct gart_domain *gart_domain;
-ÂÂÂ struct gart_device *gart;
-
-ÂÂÂ if (type != IOMMU_DOMAIN_UNMANAGED)
-ÂÂÂÂÂÂÂ return NULL;
+ÂÂÂ struct gart_device *gart = gart_handle;
 - gart = gart_handle;
-ÂÂÂ if (!gart)
+ÂÂÂ if (type != IOMMU_DOMAIN_UNMANAGED || gart->domain)
Singleton domains are a little unpleasant given the way the IOMMU API expects
things to work, but it looks fairly simple to avoid needing that at all. AFAICS
you could move gart->savedata to something like gart_domain->ptes and keep it
up-to-date in .map/.unmap, then in .attach_dev you just need to do something like:
ÂÂÂÂif (gart_domain != gart->domain) {
ÂÂÂÂÂÂÂ do_gart_setup(gart, gart_domain->ptes);
ÂÂÂÂÂÂÂ gart->domain = gart_domain;
ÂÂÂÂ}
to context-switch the hardware state when moving the group from one domain to
another (and as a bonus you would no longer need to do anything for suspend,
since resume can just look at the current domain too). If in practice there's
only ever one domain allocated anyway, then there's no difference in memory
overhead, but you still have the benefit of the driver being more consistent
with others and allowing that flexibility if anyone ever did want to play with it.
For the starter we'll have a single domain solely used by GPU with all its
sub-devices. Context switching will be handled by the Tegra's DRM driver. Later
we may consider introducing IOMMU support for the video decoder, at least to
provide memory isolation for the buffers to which decoder performs writing.
Cross-driver context switching isn't that straightforward and I think Tegra-GART
driver shouldn't take care of context switching in any form and only perform
mapping / unmapping operations. There are couple variants of how to deal with
the context switching:
1. A simple solution could be to logically split the GART's aperture space into
different domains, but GART's aperture won't be utilized efficiently with this
approach, wasting IOVA space quite a lot.
2. In order to utilize aperture more efficiently, we are going to make DRM
driver to cache IOMMU mappings such that graphics buffer will be moved to the
cache-eviction list on unmapping and actually unmapped when that buffer isn't
in-use and there is no IOVA space for another buffer or on the buffers
destruction. We'll use DRM's MM scanning helper for that [0][1]. Maybe we could
share access to that MM helper with the video decoder somehow. Seems IOMMU API
isn't tailored for a such use-case, so probably having a custom
platform-specific API on top of the IOMMU API would be fine and with that we
could have cross-device/driver context switching handled by the custom API.
Yes, if the DRM driver has overall control of the domain, then drivers
for other devices in the group are going to have to cooperate with it in
terms of IOVA allocation. It might even make sense to have that
inter-driver interface abstract things down to the map/unmap level,
since with a limited aperture you really want to avoid mapping the same
PA to two different IOVAs if at all possible.
Please let me know if you have any other variants to suggest.
[0]
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/include/drm/drm_mm.h
[1]
https://github.com/grate-driver/linux/commit/16e017efaa343e23e5a7d2d498915764cc806054
ÂÂÂÂÂÂÂÂÂ return NULL;
 - gart_domain = kzalloc(sizeof(*gart_domain), GFP_KERNEL);
-ÂÂÂ if (!gart_domain)
-ÂÂÂÂÂÂÂ return NULL;
-
-ÂÂÂ gart_domain->gart = gart;
-ÂÂÂ gart_domain->domain.geometry.aperture_start = gart->iovmm_base;
-ÂÂÂ gart_domain->domain.geometry.aperture_end = gart->iovmm_base +
+ÂÂÂ gart->domain = kzalloc(sizeof(*gart->domain), GFP_KERNEL);
+ÂÂÂ if (gart->domain) {
+ÂÂÂÂÂÂÂ gart->domain->domain.geometry.aperture_start = gart->iovmm_base;
+ÂÂÂÂÂÂÂ gart->domain->domain.geometry.aperture_end = gart->iovmm_base +
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ gart->page_count * GART_PAGE_SIZE - 1;
-ÂÂÂ gart_domain->domain.geometry.force_aperture = true;
+ÂÂÂÂÂÂÂ gart->domain->domain.geometry.force_aperture = true;
+ÂÂÂÂÂÂÂ gart->domain->gart = gart;
+ÂÂÂ }
 - return &gart_domain->domain;
+ÂÂÂ return &gart->domain->domain;
 }
  static void gart_iommu_domain_free(struct iommu_domain *domain)
@@ -251,18 +195,7 @@ static void gart_iommu_domain_free(struct iommu_domain
*domain)
ÂÂÂÂÂ struct gart_domain *gart_domain = to_gart_domain(domain);
ÂÂÂÂÂ struct gart_device *gart = gart_domain->gart;
 - if (gart) {
-ÂÂÂÂÂÂÂ spin_lock(&gart->client_lock);
-ÂÂÂÂÂÂÂ if (!list_empty(&gart->client)) {
-ÂÂÂÂÂÂÂÂÂÂÂ struct gart_client *c;
-
-ÂÂÂÂÂÂÂÂÂÂÂ list_for_each_entry(c, &gart->client, list)
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ gart_iommu_detach_dev(domain, c->dev);
-ÂÂÂÂÂÂÂ }
-ÂÂÂÂÂÂÂ spin_unlock(&gart->client_lock);
-ÂÂÂ }
-
-ÂÂÂ kfree(gart_domain);
+ÂÂÂ kfree(gart->domain);
 }
  static int gart_iommu_map(struct iommu_domain *domain, unsigned long iova,
@@ -377,7 +310,7 @@ struct iommu_group *gart_iommu_device_group(struct device
*dev)
ÂÂÂÂÂ if (err)
ÂÂÂÂÂÂÂÂÂ return ERR_PTR(err);
 - return generic_device_group(dev);
+ÂÂÂ return gart_handle->group;
You should take a reference per device, i.e.:
ÂÂÂÂreturn iommu_group_ref_get(gart_handle->group);
otherwise removing devices could unbalance things and result in the group
getting freed prematurely.
Seems more correctly would be to remove iommu_group_put() from
gart_iommu_add_device().
If you're confident that no bus code will ever result in add_device()
getting called more than once for the same device, then you could get
away with that. AFAIK it *shouldn't* happen, but I've never managed to
convince myself that it *can't*.
Robin.
 }
  static int gart_iommu_of_xlate(struct device *dev,
@@ -502,8 +435,6 @@ static int tegra_gart_probe(struct platform_device *pdev)
 Â gart->dev = &pdev->dev;
ÂÂÂÂÂ spin_lock_init(&gart->pte_lock);
-ÂÂÂ spin_lock_init(&gart->client_lock);
-ÂÂÂ INIT_LIST_HEAD(&gart->client);
ÂÂÂÂÂ gart->regs = gart_regs;
ÂÂÂÂÂ gart->iovmm_base = (dma_addr_t)res_remap->start;
ÂÂÂÂÂ gart->page_count = (resource_size(res_remap) >> GART_PAGE_SHIFT);
@@ -517,6 +448,14 @@ static int tegra_gart_probe(struct platform_device *pdev)
ÂÂÂÂÂÂÂÂÂ goto iommu_unregister;
ÂÂÂÂÂ }
 + gart->group = iommu_group_alloc();
+ÂÂÂ if (IS_ERR(gart->group)) {
+ÂÂÂÂÂÂÂ ret = PTR_ERR(gart->group);
+ÂÂÂÂÂÂÂ goto free_savedata;
+ÂÂÂ }
+
+ÂÂÂ iommu_group_ref_get(gart->group);
You already hold the initial reference from iommu_group_alloc(), so there's no
need to take a second one at this point.
Yes, looks like this refcount-bump isn't needed here. I'll revisit the
refcountings and correct them in v2 where necessary.
Thank you very much for the review.