tools/testing/nvdimm: libnvdimm unit test infrastructure

'libnvdimm' is the first driver sub-system in the kernel to implement
mocking for unit test coverage.  The nfit_test module gets built as an
external module and arranges for external module replacements of nfit,
libnvdimm, nd_pmem, and nd_blk.  These replacements use the linker
--wrap option to redirect calls to ioremap() + request_mem_region() to
custom defined unit test resources.  The end result is a fully
functional nvdimm_bus, as far as userspace is concerned, but with the
capability to perform otherwise destructive tests on emulated resources.

Q: Why not use QEMU for this emulation?
QEMU is not suitable for unit testing.  QEMU's role is to faithfully
emulate the platform.  A unit test's role is to unfaithfully implement
the platform with the goal of triggering bugs in the corners of the
sub-system implementation.  As bugs are discovered in platforms, or the
sub-system itself, the unit tests are extended to backstop a fix with a
reproducer unit test.

Another problem with QEMU is that it would require coordination of 3
software projects instead of 2 (kernel + libndctl [1]) to maintain and
execute the tests.  The chances for bit rot and the difficulty of
getting the tests running goes up non-linearly the more components
involved.


Q: Why submit this to the kernel tree instead of external modules in
   libndctl?
Simple, to alleviate the same risk that out-of-tree external modules
face.  Updates to drivers/nvdimm/ can be immediately evaluated to see if
they have any impact on tools/testing/nvdimm/.


Q: What are the negative implications of merging this?
It is a unique maintenance burden because the purpose of mocking an
interface to enable a unit test is to purposefully short circuit the
semantics of a routine to enable testing.  For example
__wrap_ioremap_cache() fakes the pmem driver into "ioremap()'ing" a test
resource buffer allocated by dma_alloc_coherent().  The future
maintenance burden hits when someone changes the semantics of
ioremap_cache() and wonders what the implications are for the unit test.

[1]: https://github.com/pmem/ndctl

Cc: <linux-acpi@vger.kernel.org>
Cc: Lv Zheng <lv.zheng@intel.com>
Cc: Robert Moore <robert.moore@intel.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/acpi/nfit.c

@@ -33,10 +33,11 @@ MODULE_PARM_DESC(force_enable_dimms, "Ignore _STA (ACPI DIMM device) status");
 
 static u8 nfit_uuid[NFIT_UUID_MAX][16];
 
-static const u8 *to_nfit_uuid(enum nfit_uuids id)
+const u8 *to_nfit_uuid(enum nfit_uuids id)
 {
 	return nfit_uuid[id];
 }
+EXPORT_SYMBOL(to_nfit_uuid);
 
 static struct acpi_nfit_desc *to_acpi_nfit_desc(
 		struct nvdimm_bus_descriptor *nd_desc)
@@ -581,11 +582,12 @@ static struct attribute_group acpi_nfit_attribute_group = {
 	.attrs = acpi_nfit_attributes,
 };
 
-static const struct attribute_group *acpi_nfit_attribute_groups[] = {
+const struct attribute_group *acpi_nfit_attribute_groups[] = {
 	&nvdimm_bus_attribute_group,
 	&acpi_nfit_attribute_group,
 	NULL,
 };
+EXPORT_SYMBOL_GPL(acpi_nfit_attribute_groups);
 
 static struct acpi_nfit_memory_map *to_nfit_memdev(struct device *dev)
 {
@@ -1323,7 +1325,7 @@ static int acpi_nfit_init_mapping(struct acpi_nfit_desc *acpi_desc,
 		ndbr_desc = to_blk_region_desc(ndr_desc);
 		ndbr_desc->enable = acpi_nfit_blk_region_enable;
 		ndbr_desc->disable = acpi_nfit_blk_region_disable;
-		ndbr_desc->do_io = acpi_nfit_blk_region_do_io;
+		ndbr_desc->do_io = acpi_desc->blk_do_io;
 		if (!nvdimm_blk_region_create(acpi_desc->nvdimm_bus, ndr_desc))
 			return -ENOMEM;
 		break;
@@ -1407,7 +1409,7 @@ static int acpi_nfit_register_regions(struct acpi_nfit_desc *acpi_desc)
 	return 0;
 }
 
-static int acpi_nfit_init(struct acpi_nfit_desc *acpi_desc, acpi_size sz)
+int acpi_nfit_init(struct acpi_nfit_desc *acpi_desc, acpi_size sz)
 {
 	struct device *dev = acpi_desc->dev;
 	const void *end;
@@ -1446,6 +1448,7 @@ static int acpi_nfit_init(struct acpi_nfit_desc *acpi_desc, acpi_size sz)
 
 	return acpi_nfit_register_regions(acpi_desc);
 }
+EXPORT_SYMBOL_GPL(acpi_nfit_init);
 
 static int acpi_nfit_add(struct acpi_device *adev)
 {
@@ -1470,6 +1473,7 @@ static int acpi_nfit_add(struct acpi_device *adev)
 	dev_set_drvdata(dev, acpi_desc);
 	acpi_desc->dev = dev;
 	acpi_desc->nfit = (struct acpi_table_nfit *) tbl;
+	acpi_desc->blk_do_io = acpi_nfit_blk_region_do_io;
 	nd_desc = &acpi_desc->nd_desc;
 	nd_desc->provider_name = "ACPI.NFIT";
 	nd_desc->ndctl = acpi_nfit_ctl;