iommu/tegra-smmu: Rework tegra_smmu_probe_device()

The bus_set_iommu() in tegra_smmu_probe() enumerates all clients
to call in tegra_smmu_probe_device() where each client searches
its DT node for smmu pointer and swgroup ID, so as to configure
an fwspec. But this requires a valid smmu pointer even before mc
and smmu drivers are probed. So in tegra_smmu_probe() we added a
line of code to fill mc->smmu, marking "a bit of a hack".

This works for most of clients in the DTB, however, doesn't work
for a client that doesn't exist in DTB, a PCI device for example.

Actually, if we return ERR_PTR(-ENODEV) in ->probe_device() when
it's called from bus_set_iommu(), iommu core will let everything
carry on. Then when a client gets probed, of_iommu_configure() in
iommu core will search DTB for swgroup ID and call ->of_xlate()
to prepare an fwspec, similar to tegra_smmu_probe_device() and
tegra_smmu_configure(). Then it'll call tegra_smmu_probe_device()
again, and this time we shall return smmu->iommu pointer properly.

So we can get rid of tegra_smmu_find() and tegra_smmu_configure()
along with DT polling code by letting the iommu core handle every
thing, except a problem that we search iommus property in DTB not
only for swgroup ID but also for mc node to get mc->smmu pointer
to call dev_iommu_priv_set() and return the smmu->iommu pointer.
So we'll need to find another way to get smmu pointer.

Referencing the implementation of sun50i-iommu driver, of_xlate()
has client's dev pointer, mc node and swgroup ID. This means that
we can call dev_iommu_priv_set() in of_xlate() instead, so we can
simply get smmu pointer in ->probe_device().

This patch reworks tegra_smmu_probe_device() by:
1) Removing mc->smmu hack in tegra_smmu_probe() so as to return
   ERR_PTR(-ENODEV) in tegra_smmu_probe_device() during stage of
   tegra_smmu_probe/tegra_mc_probe().
2) Moving dev_iommu_priv_set() to of_xlate() so we can get smmu
   pointer in tegra_smmu_probe_device() to replace DTB polling.
3) Removing tegra_smmu_configure() accordingly since iommu core
   takes care of it.

This also fixes a problem that previously we could add clients to
iommu groups before iommu core initializes its default domain:
    ubuntu@jetson:~$ dmesg | grep iommu
    platform 50000000.host1x: Adding to iommu group 1
    platform 57000000.gpu: Adding to iommu group 2
    iommu: Default domain type: Translated
    platform 54200000.dc: Adding to iommu group 3
    platform 54240000.dc: Adding to iommu group 3
    platform 54340000.vic: Adding to iommu group 4

Though it works fine with IOMMU_DOMAIN_UNMANAGED, but will have
warnings if switching to IOMMU_DOMAIN_DMA:
    iommu: Failed to allocate default IOMMU domain of type 0 for
           group (null) - Falling back to IOMMU_DOMAIN_DMA
    iommu: Failed to allocate default IOMMU domain of type 0 for
           group (null) - Falling back to IOMMU_DOMAIN_DMA

Now, bypassing the first probe_device() call from bus_set_iommu()
fixes the sequence:
    ubuntu@jetson:~$ dmesg | grep iommu
    iommu: Default domain type: Translated
    tegra-host1x 50000000.host1x: Adding to iommu group 0
    tegra-dc 54200000.dc: Adding to iommu group 1
    tegra-dc 54240000.dc: Adding to iommu group 1
    tegra-vic 54340000.vic: Adding to iommu group 2
    nouveau 57000000.gpu: Adding to iommu group 3

Note that dmesg log above is testing with IOMMU_DOMAIN_UNMANAGED.
Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20201125101013.14953-5-nicoleotsuka@gmail.comSigned-off-by: Will Deacon <will@kernel.org>

drivers/iommu/tegra-smmu.c

@@ -797,75 +797,9 @@ static phys_addr_t tegra_smmu_iova_to_phys(struct iommu_domain *domain,
 	return SMMU_PFN_PHYS(pfn) + SMMU_OFFSET_IN_PAGE(iova);
 }
 
-static struct tegra_smmu *tegra_smmu_find(struct device_node *np)
-{
-	struct platform_device *pdev;
-	struct tegra_mc *mc;
-
-	pdev = of_find_device_by_node(np);
-	if (!pdev)
-		return NULL;
-
-	mc = platform_get_drvdata(pdev);
-	if (!mc)
-		return NULL;
-
-	return mc->smmu;
-}
-
-static int tegra_smmu_configure(struct tegra_smmu *smmu, struct device *dev,
-				struct of_phandle_args *args)
-{
-	const struct iommu_ops *ops = smmu->iommu.ops;
-	int err;
-
-	err = iommu_fwspec_init(dev, &dev->of_node->fwnode, ops);
-	if (err < 0) {
-		dev_err(dev, "failed to initialize fwspec: %d\n", err);
-		return err;
-	}
-
-	err = ops->of_xlate(dev, args);
-	if (err < 0) {
-		dev_err(dev, "failed to parse SW group ID: %d\n", err);
-		iommu_fwspec_free(dev);
-		return err;
-	}
-
-	return 0;
-}
-
 static struct iommu_device *tegra_smmu_probe_device(struct device *dev)
 {
-	struct device_node *np = dev->of_node;
-	struct tegra_smmu *smmu = NULL;
-	struct of_phandle_args args;
-	unsigned int index = 0;
-	int err;
-
-	while (of_parse_phandle_with_args(np, "iommus", "#iommu-cells", index,
-					  &args) == 0) {
-		smmu = tegra_smmu_find(args.np);
-		if (smmu) {
-			err = tegra_smmu_configure(smmu, dev, &args);
-			of_node_put(args.np);
-
-			if (err < 0)
-				return ERR_PTR(err);
-
-			/*
-			 * Only a single IOMMU master interface is currently
-			 * supported by the Linux kernel, so abort after the
-			 * first match.
-			 */
-			dev_iommu_priv_set(dev, smmu);
-
-			break;
-		}
-
-		of_node_put(args.np);
-		index++;
-	}
+	struct tegra_smmu *smmu = dev_iommu_priv_get(dev);
 
 	if (!smmu)
 		return ERR_PTR(-ENODEV);
@@ -873,10 +807,7 @@ static struct iommu_device *tegra_smmu_probe_device(struct device *dev)
 	return &smmu->iommu;
 }
 
-static void tegra_smmu_release_device(struct device *dev)
-{
-	dev_iommu_priv_set(dev, NULL);
-}
+static void tegra_smmu_release_device(struct device *dev) {}
 
 static const struct tegra_smmu_group_soc *
 tegra_smmu_find_group(struct tegra_smmu *smmu, unsigned int swgroup)
@@ -953,8 +884,21 @@ static struct iommu_group *tegra_smmu_device_group(struct device *dev)
 static int tegra_smmu_of_xlate(struct device *dev,
 			       struct of_phandle_args *args)
 {
+	struct platform_device *iommu_pdev = of_find_device_by_node(args->np);
+	struct tegra_mc *mc = platform_get_drvdata(iommu_pdev);
 	u32 id = args->args[0];
 
+	/*
+	 * Note: we are here releasing the reference of &iommu_pdev->dev, which
+	 * is mc->dev. Although some functions in tegra_smmu_ops may keep using
+	 * its private data beyond this point, it's still safe to do so because
+	 * the SMMU parent device is the same as the MC, so the reference count
+	 * isn't strictly necessary.
+	 */
+	put_device(&iommu_pdev->dev);
+
+	dev_iommu_priv_set(dev, mc->smmu);
+
 	return iommu_fwspec_add_ids(dev, &id, 1);
 }
 
@@ -1079,16 +1023,6 @@ struct tegra_smmu *tegra_smmu_probe(struct device *dev,
 	if (!smmu)
 		return ERR_PTR(-ENOMEM);
 
-	/*
-	 * This is a bit of a hack. Ideally we'd want to simply return this
-	 * value. However the IOMMU registration process will attempt to add
-	 * all devices to the IOMMU when bus_set_iommu() is called. In order
-	 * not to rely on global variables to track the IOMMU instance, we
-	 * set it here so that it can be looked up from the .probe_device()
-	 * callback via the IOMMU device's .drvdata field.
-	 */
-	mc->smmu = smmu;
-
 	size = BITS_TO_LONGS(soc->num_asids) * sizeof(long);
 
 	smmu->asids = devm_kzalloc(dev, size, GFP_KERNEL);