gpu: host1x: Add context bus

The context bus is a "dummy" bus that contains struct devices that
correspond to IOMMU contexts assigned through Host1x to processes.

Even when host1x itself is built as a module, the bus is registered
in built-in code so that the built-in ARM SMMU driver is able to
reference it.
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/Makefile

@@ -2,7 +2,6 @@
 # drm/tegra depends on host1x, so if both drivers are built-in care must be
 # taken to initialize them in the correct order. Link order is the only way
 # to ensure this currently.
-obj-$(CONFIG_TEGRA_HOST1X)	+= host1x/
-obj-y			+= drm/ vga/
+obj-y			+= host1x/ drm/ vga/
 obj-$(CONFIG_IMX_IPUV3_CORE)	+= ipu-v3/
 obj-$(CONFIG_TRACE_GPU_MEM)		+= trace/

drivers/gpu/host1x/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
+
+config TEGRA_HOST1X_CONTEXT_BUS
+	bool
+
 config TEGRA_HOST1X
 	tristate "NVIDIA Tegra host1x driver"
 	depends on ARCH_TEGRA || (ARM && COMPILE_TEST)
 	select DMA_SHARED_BUFFER
+	select TEGRA_HOST1X_CONTEXT_BUS
 	select IOMMU_IOVA
 	help
 	  Driver for the NVIDIA Tegra host1x hardware.

drivers/gpu/host1x/Makefile

@@ -18,3 +18,4 @@ host1x-y = \
 	hw/host1x07.o
 
 obj-$(CONFIG_TEGRA_HOST1X) += host1x.o
+obj-$(CONFIG_TEGRA_HOST1X_CONTEXT_BUS) += context_bus.o

drivers/gpu/host1x/context_bus.c

+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2021, NVIDIA Corporation.
+ */
+
+#include <linux/device.h>
+#include <linux/of.h>
+
+struct bus_type host1x_context_device_bus_type = {
+	.name = "host1x-context",
+};
+EXPORT_SYMBOL_GPL(host1x_context_device_bus_type);
+
+static int __init host1x_context_device_bus_init(void)
+{
+	int err;
+
+	if (!of_machine_is_compatible("nvidia,tegra186") &&
+	    !of_machine_is_compatible("nvidia,tegra194") &&
+	    !of_machine_is_compatible("nvidia,tegra234"))
+		return 0;
+
+	err = bus_register(&host1x_context_device_bus_type);
+	if (err < 0) {
+		pr_err("bus type registration failed: %d\n", err);
+		return err;
+	}
+
+	return 0;
+}
+postcore_initcall(host1x_context_device_bus_init);

include/linux/host1x_context_bus.h

+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (c) 2021, NVIDIA Corporation. All rights reserved.
+ */
+
+#ifndef __LINUX_HOST1X_CONTEXT_BUS_H
+#define __LINUX_HOST1X_CONTEXT_BUS_H
+
+#include <linux/device.h>
+
+#ifdef CONFIG_TEGRA_HOST1X_CONTEXT_BUS
+extern struct bus_type host1x_context_device_bus_type;
+#endif
+
+#endif