[RFC PATCH 5/5] device-dax: Add a driver for "hmem" devices

[Dan Williams]

Platform firmware like EFI/ACPI may publish "hmem" platform devices.
Such a device is a performance differentiated memory range likely
reserved for an application specific use case. The driver gives access
to 100% of the capacity via a device-dax mmap instance by default.

However, if over-subscription and other kernel memory management is
desired the resulting dax device can be assigned to the core-mm via the
kmem driver.

Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
Cc: Keith Busch <keith.busch@xxxxxxxxx>
Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 drivers/dax/Kconfig  |   26 ++++++++++++++++++----
 drivers/dax/Makefile |    2 ++
 drivers/dax/hmem.c   |   58 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 81 insertions(+), 5 deletions(-)
 create mode 100644 drivers/dax/hmem.c

diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index 5ef624fe3934..b3886f43dd77 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -38,16 +38,32 @@ config DEV_DAX_KMEM
 	depends on DEV_DAX
 	depends on MEMORY_HOTPLUG # for add_memory() and friends
 	help
-	  Support access to persistent memory as if it were RAM.  This
-	  allows easier use of persistent memory by unmodified
-	  applications.
+	  Support access to persistent, or other performance
+	  differentiated memory as if it were System RAM. This allows
+	  easier use of persistent memory by unmodified applications, or
+	  adds core kernel memory services to heterogeneous memory types
+	  (HMEM) marked "reserved" by platform firmware.
 
 	  To use this feature, a DAX device must be unbound from the
-	  device_dax driver (PMEM DAX) and bound to this kmem driver
-	  on each boot.
+	  device_dax driver and bound to this kmem driver on each boot.
 
 	  Say N if unsure.
 
+config DEV_DAX_HMEM
+	tristate "HMEM DAX: generic support for 'special purpose' memory"
+	default DEV_DAX
+	help
+	  EFI 2.8 platforms, and others, may advertise 'special purpose'
+	  memory.  For example, a high bandwidth memory pool. The
+	  indication from platform firmware is meant to reserve the
+	  memory from typical usage by default.  This driver creates
+	  device-dax instances for these memory ranges, and that also
+	  enables the possibility to assign them to the DEV_DAX_KMEM
+	  driver to override the reservation and add them to kernel
+	  "System RAM" pool.
+
+	  Say Y if unsure.
+
 config DEV_DAX_PMEM_COMPAT
 	tristate "PMEM DAX: support the deprecated /sys/class/dax interface"
 	depends on DEV_DAX_PMEM
diff --git a/drivers/dax/Makefile b/drivers/dax/Makefile
index 81f7d54dadfb..80065b38b3c4 100644
--- a/drivers/dax/Makefile
+++ b/drivers/dax/Makefile
@@ -2,9 +2,11 @@
 obj-$(CONFIG_DAX) += dax.o
 obj-$(CONFIG_DEV_DAX) += device_dax.o
 obj-$(CONFIG_DEV_DAX_KMEM) += kmem.o
+obj-$(CONFIG_DEV_DAX_HMEM) += dax_hmem.o
 
 dax-y := super.o
 dax-y += bus.o
 device_dax-y := device.o
+dax_hmem-y := hmem.o
 
 obj-y += pmem/
diff --git a/drivers/dax/hmem.c b/drivers/dax/hmem.c
new file mode 100644
index 000000000000..b01cf9c65329
--- /dev/null
+++ b/drivers/dax/hmem.c
@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/platform_device.h>
+#include <linux/memregion.h>
+#include <linux/memremap.h>
+#include <linux/module.h>
+#include <linux/pfn_t.h>
+#include "bus.h"
+
+static int dax_hmem_probe(struct platform_device *pdev)
+{
+	struct dev_pagemap pgmap = { 0 };
+	struct device *dev = &pdev->dev;
+	struct dax_region *dax_region;
+	struct memregion_info *mri;
+	struct dev_dax *dev_dax;
+	struct resource *res;
+
+	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
+	if (!res)
+		return -ENOMEM;
+
+	mri = dev->platform_data;
+	pgmap.dev = dev;
+	memcpy(&pgmap.res, res, sizeof(*res));
+
+	dax_region = alloc_dax_region(dev, pdev->id, res, mri->target_node,
+			PMD_SIZE, PFN_DEV|PFN_MAP);
+	if (!dax_region)
+		return -ENOMEM;
+
+	dev_dax = devm_create_dev_dax(dax_region, 0, &pgmap);
+	if (IS_ERR(dev_dax))
+		return PTR_ERR(dev_dax);
+
+	/* child dev_dax instances now own the lifetime of the dax_region */
+	dax_region_put(dax_region);
+	return 0;
+}
+
+static int dax_hmem_remove(struct platform_device *pdev)
+{
+	/* devm handles teardown */
+	return 0;
+}
+
+static struct platform_driver dax_hmem_driver = {
+	.probe = dax_hmem_probe,
+	.remove = dax_hmem_remove,
+	.driver = {
+		.name = "hmem",
+	},
+};
+
+module_platform_driver(dax_hmem_driver);
+
+MODULE_ALIAS("platform:hmem*");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Intel Corporation");