lightnvm: deprecated OCSSD support and schedule it for removal in Linux 5.15

Lightnvm was an innovative idea to expose more low-level control over SSDs.
But it failed to get properly standardized and remains a non-standarized
extension to NVMe that requires vendor specific quirks for a few now mostly
obsolete SSD devices.  The standardized ZNS command set for NVMe has take
over a lot of the approaches and allows for fully standardized operation.

Remove the Linux code to support open channel SSDs as the few production
deployments of the above mentioned SSDs are using userspace driver stacks
instead of the fairly limited Linux support.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Link: https://lore.kernel.org/r/20210413105257.159260-5-matias.bjorling@wdc.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/lightnvm/Kconfig

@@ -4,7 +4,7 @@
 #
 
 menuconfig NVM
-	bool "Open-Channel SSD target support"
+	bool "Open-Channel SSD target support (DEPRECATED)"
 	depends on BLOCK
 	help
 	  Say Y here to get to enable Open-channel SSDs.
@@ -15,6 +15,8 @@ menuconfig NVM
 	  If you say N, all options in this submenu will be skipped and disabled
 	  only do this if you know what you are doing.
 
+	  This code is deprecated and will be removed in Linux 5.15.
+
 if NVM
 
 config NVM_PBLK

drivers/lightnvm/core.c

@@ -1174,6 +1174,8 @@ int nvm_register(struct nvm_dev *dev)
 {
 	int ret, exp_pool_size;
 
+	pr_warn_once("lightnvm support is deprecated and will be removed in Linux 5.15.\n");
+
 	if (!dev->q || !dev->ops) {
 		kref_put(&dev->ref, nvm_free);
 		return -EINVAL;