scsi: core: Introduce {init,exit}_cmd_priv()

The current behavior of the SCSI core is to clear driver-private data
before preparing a request for submission to the SCSI LLD. Make it possible
for SCSI LLDs to disable clearing of driver-private data.

These hooks will be used by a later patch, namely "scsi: ufs: Let the SCSI
core allocate per-command UFS data".

Link: https://lore.kernel.org/r/20200123035637.21848-2-bvanassche@acm.org
Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: Stanley Chu <stanley.chu@mediatek.com>
Cc: Bean Huo <beanhuo@micron.com>
Cc: Avri Altman <avri.altman@wdc.com>
Cc: Can Guo <cang@codeaurora.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Johannes Thumshirn <jth@kernel.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_lib.c

@@ -1097,15 +1097,15 @@ static void scsi_cleanup_rq(struct request *rq)
 	}
 }
 
-/* Called after a request has been started. */
+/* Called before a request is prepared. See also scsi_mq_prep_fn(). */
 void scsi_init_command(struct scsi_device *dev, struct scsi_cmnd *cmd)
 {
 	void *buf = cmd->sense_buffer;
 	void *prot = cmd->prot_sdb;
 	struct request *rq = blk_mq_rq_from_pdu(cmd);
 	unsigned int flags = cmd->flags & SCMD_PRESERVED_FLAGS;
 	unsigned long jiffies_at_alloc;
-	int retries;
+	int retries, to_clear;
 	bool in_flight;
 
 	if (!blk_rq_is_scsi(rq) && !(flags & SCMD_INITIALIZED)) {
@@ -1116,9 +1116,15 @@ void scsi_init_command(struct scsi_device *dev, struct scsi_cmnd *cmd)
 	jiffies_at_alloc = cmd->jiffies_at_alloc;
 	retries = cmd->retries;
 	in_flight = test_bit(SCMD_STATE_INFLIGHT, &cmd->state);
-	/* zero out the cmd, except for the embedded scsi_request */
-	memset((char *)cmd + sizeof(cmd->req), 0,
-		sizeof(*cmd) - sizeof(cmd->req) + dev->host->hostt->cmd_size);
+	/*
+	 * Zero out the cmd, except for the embedded scsi_request. Only clear
+	 * the driver-private command data if the LLD does not supply a
+	 * function to initialize that data.
+	 */
+	to_clear = sizeof(*cmd) - sizeof(cmd->req);
+	if (!dev->host->hostt->init_cmd_priv)
+		to_clear += dev->host->hostt->cmd_size;
+	memset((char *)cmd + sizeof(cmd->req), 0, to_clear);
 
 	cmd->device = dev;
 	cmd->sense_buffer = buf;
@@ -1711,6 +1717,7 @@ static int scsi_mq_init_request(struct blk_mq_tag_set *set, struct request *rq,
 	const bool unchecked_isa_dma = shost->unchecked_isa_dma;
 	struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(rq);
 	struct scatterlist *sg;
+	int ret = 0;
 
 	if (unchecked_isa_dma)
 		cmd->flags |= SCMD_UNCHECKED_ISA_DMA;
@@ -1726,14 +1733,24 @@ static int scsi_mq_init_request(struct blk_mq_tag_set *set, struct request *rq,
 		cmd->prot_sdb = (void *)sg + scsi_mq_inline_sgl_size(shost);
 	}
 
-	return 0;
+	if (shost->hostt->init_cmd_priv) {
+		ret = shost->hostt->init_cmd_priv(shost, cmd);
+		if (ret < 0)
+			scsi_free_sense_buffer(unchecked_isa_dma,
+					       cmd->sense_buffer);
+	}
+
+	return ret;
 }
 
 static void scsi_mq_exit_request(struct blk_mq_tag_set *set, struct request *rq,
 				 unsigned int hctx_idx)
 {
+	struct Scsi_Host *shost = set->driver_data;
 	struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(rq);
 
+	if (shost->hostt->exit_cmd_priv)
+		shost->hostt->exit_cmd_priv(shost, cmd);
 	scsi_free_sense_buffer(cmd->flags & SCMD_UNCHECKED_ISA_DMA,
 			       cmd->sense_buffer);
 }

include/scsi/scsi_host.h

@@ -62,6 +62,9 @@ struct scsi_host_template {
 			    void __user *arg);
 #endif
 
+	int (*init_cmd_priv)(struct Scsi_Host *shost, struct scsi_cmnd *cmd);
+	int (*exit_cmd_priv)(struct Scsi_Host *shost, struct scsi_cmnd *cmd);
+
 	/*
 	 * The queuecommand function is used to queue up a scsi
 	 * command block to the LLDD.  When the driver finished