[RFC PATCH 3/4] mm/memory_hotplug: Add max_configurable sysfs read attribute

[Sumanth Korikkar <sumanthk@linux.ibm.com>]

Introduce /sys/devices/system/memory/max_configurable sysfs interface to
list maximum number of configurable memory blocks by the architecture.
Using this information, lsmem tool could possibly read memory blocks
/sys/devices/system/memory/memory[0-9]+ as configured and list rest of
the max_configurable count as deconfigured.

Arch can define max_configurable by overriding
arch_get_memory_max_configurable().

s390 usecase:
/sys/devices/system/memory/max_configurable would be online + standby
memory blocks count. The max_configurable attribute for s390 is
configured in the next patch.

x86 possible usecase:
max_configurable could be initially set to count of all online memory
blocks. Later, max_configurable can be updated whenever there is an
acpi event generation for hotplugged memory. Userspace application like
lsmem could possibly read memory blocks
/sys/devices/system/memory/memory[0-9]+ as configured and rest of the
max_configurable count as deconfigured.

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
---
 drivers/base/memory.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 469adc7212fc..3da83345b570 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -665,6 +665,15 @@ bool __weak arch_validate_memory_range(unsigned long long start,
 	return false;
 }
 
+/*
+ * Arch can override the function and return the number of maximum configurable
+ * memory.
+ */
+ssize_t __weak arch_get_memory_max_configurable(void)
+{
+	return 0;
+}
+
 /*
  * Format:
  * echo config_mode,memoryrange,altmap_mode >
@@ -759,6 +768,15 @@ static ssize_t configure_memory_store(struct device *dev,
 	return ret ? ret : count;
 }
 static DEVICE_ATTR_WO(configure_memory);
+
+/*
+ * Show the maximum number of configurable memory.
+ */
+static ssize_t max_configurable_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%lu\n", arch_get_memory_max_configurable());
+}
+static DEVICE_ATTR_RO(max_configurable);
 #endif /* CONFIG_RUNTIME_MEMORY_CONFIGURATION */
 
 /*
@@ -1075,6 +1093,7 @@ static struct attribute *memory_root_attrs[] = {
 #endif
 #ifdef CONFIG_RUNTIME_MEMORY_CONFIGURATION
 	&dev_attr_configure_memory.attr,
+	&dev_attr_max_configurable.attr,
 #endif
 	NULL
 };
-- 
2.45.2