null_blk: add module parameters for 4 options

Add as module parameters these options:

memory_backed
discard
mbps
cache_size

Previously these could only be set via configfs.

Still missing is bad_blocks.

The kernel test robot found a documentation formatting issue in v1 of
this patch.

Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Vincent Fu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/block/null_blk.rst

···
 hw_queue_depth=[0..qdepth]: Default: 64
   The hardware queue depth of the device.
 
+memory_backed=[0/1]: Default: 0
+  Whether or not to use a memory buffer to respond to IO requests
+
+  =  =============================================
+  0  Transfer no data in response to IO requests
+  1  Use a memory buffer to respond to IO requests
+  =  =============================================
+
+discard=[0/1]: Default: 0
+  Support discard operations (requires memory-backed null_blk device).
+
+  =  =====================================
+  0  Do not support discard operations
+  1  Enable support for discard operations
+  =  =====================================
+
+cache_size=[Size in MB]: Default: 0
+  Cache size in MB for memory-backed device.
+
+mbps=[Maximum bandwidth in MB/s]: Default: 0 (no limit)
+  Bandwidth limit for device performance.
+
 Multi-queue specific parameters
 -------------------------------
 

drivers/block/null_blk/main.c

···
 module_param_named(use_per_node_hctx, g_use_per_node_hctx, bool, 0444);
 MODULE_PARM_DESC(use_per_node_hctx, "Use per-node allocation for hardware context queues. Default: false");
 
+static bool g_memory_backed;
+module_param_named(memory_backed, g_memory_backed, bool, 0444);
+MODULE_PARM_DESC(memory_backed, "Create a memory-backed block device. Default: false");
+
+static bool g_discard;
+module_param_named(discard, g_discard, bool, 0444);
+MODULE_PARM_DESC(discard, "Support discard operations (requires memory-backed null_blk device). Default: false");
+
+static unsigned long g_cache_size;
+module_param_named(cache_size, g_cache_size, ulong, 0444);
+MODULE_PARM_DESC(mbps, "Cache size in MiB for memory-backed device. Default: 0 (none)");
+
+static unsigned int g_mbps;
+module_param_named(mbps, g_mbps, uint, 0444);
+MODULE_PARM_DESC(mbps, "Limit maximum bandwidth (in MiB/s). Default: 0 (no limit)");
+
 static bool g_zoned;
 module_param_named(zoned, g_zoned, bool, S_IRUGO);
 MODULE_PARM_DESC(zoned, "Make device as a host-managed zoned block device. Default: false");
···
 	dev->irqmode = g_irqmode;
 	dev->hw_queue_depth = g_hw_queue_depth;
 	dev->blocking = g_blocking;
+	dev->memory_backed = g_memory_backed;
+	dev->discard = g_discard;
+	dev->cache_size = g_cache_size;
+	dev->mbps = g_mbps;
 	dev->use_per_node_hctx = g_use_per_node_hctx;
 	dev->zoned = g_zoned;
 	dev->zone_size = g_zone_size;