scsi: esp: use sg helper to iterate over scatterlist

Unlike the legacy I/O path, scsi-mq preallocates a large array to hold
the scatterlist for each request. This static allocation can consume
substantial amounts of memory on modern controllers which support a
large number of concurrently outstanding requests.

To facilitate a switch to a smaller static allocation combined with a
dynamic allocation for requests that need it, we need to make sure all
SCSI drivers handle chained scatterlists correctly.

Convert remaining drivers that directly dereference the scatterlist
array to using the iterator functions.

[mkp: clarified commit message]

Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Finn Thain <fthain@telegraphics.com.au>
Cc: Guenter Roeck <linux@roeck-us.net>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/esp_scsi.c

@@ -370,6 +370,7 @@ static void esp_map_dma(struct esp *esp, struct scsi_cmnd *cmd)
 	struct esp_cmd_priv *spriv = ESP_CMD_PRIV(cmd);
 	struct scatterlist *sg = scsi_sglist(cmd);
 	int total = 0, i;
+	struct scatterlist *s;
 
 	if (cmd->sc_data_direction == DMA_NONE)
 		return;
@@ -380,16 +381,18 @@ static void esp_map_dma(struct esp *esp, struct scsi_cmnd *cmd)
 		 * a dma address, so perform an identity mapping.
 		 */
 		spriv->num_sg = scsi_sg_count(cmd);
-		for (i = 0; i < spriv->num_sg; i++) {
-			sg[i].dma_address = (uintptr_t)sg_virt(&sg[i]);
-			total += sg_dma_len(&sg[i]);
+
+		scsi_for_each_sg(cmd, s, spriv->num_sg, i) {
+			s->dma_address = (uintptr_t)sg_virt(s);
+			total += sg_dma_len(s);
 		}
 	} else {
 		spriv->num_sg = scsi_dma_map(cmd);
-		for (i = 0; i < spriv->num_sg; i++)
-			total += sg_dma_len(&sg[i]);
+		scsi_for_each_sg(cmd, s, spriv->num_sg, i)
+			total += sg_dma_len(s);
 	}
 	spriv->cur_residue = sg_dma_len(sg);
+	spriv->prv_sg = NULL;
 	spriv->cur_sg = sg;
 	spriv->tot_residue = total;
 }
@@ -443,7 +446,8 @@ static void esp_advance_dma(struct esp *esp, struct esp_cmd_entry *ent,
 		p->tot_residue = 0;
 	}
 	if (!p->cur_residue && p->tot_residue) {
-		p->cur_sg++;
+		p->prv_sg = p->cur_sg;
+		p->cur_sg = sg_next(p->cur_sg);
 		p->cur_residue = sg_dma_len(p->cur_sg);
 	}
 }
@@ -464,6 +468,7 @@ static void esp_save_pointers(struct esp *esp, struct esp_cmd_entry *ent)
 		return;
 	}
 	ent->saved_cur_residue = spriv->cur_residue;
+	ent->saved_prv_sg = spriv->prv_sg;
 	ent->saved_cur_sg = spriv->cur_sg;
 	ent->saved_tot_residue = spriv->tot_residue;
 }
@@ -478,6 +483,7 @@ static void esp_restore_pointers(struct esp *esp, struct esp_cmd_entry *ent)
 		return;
 	}
 	spriv->cur_residue = ent->saved_cur_residue;
+	spriv->prv_sg = ent->saved_prv_sg;
 	spriv->cur_sg = ent->saved_cur_sg;
 	spriv->tot_residue = ent->saved_tot_residue;
 }
@@ -1646,7 +1652,7 @@ static int esp_msgin_process(struct esp *esp)
 		spriv = ESP_CMD_PRIV(ent->cmd);
 
 		if (spriv->cur_residue == sg_dma_len(spriv->cur_sg)) {
-			spriv->cur_sg--;
+			spriv->cur_sg = spriv->prv_sg;
 			spriv->cur_residue = 1;
 		} else
 			spriv->cur_residue++;

drivers/scsi/esp_scsi.h

@@ -251,6 +251,7 @@
 struct esp_cmd_priv {
 	int			num_sg;
 	int			cur_residue;
+	struct scatterlist	*prv_sg;
 	struct scatterlist	*cur_sg;
 	int			tot_residue;
 };
@@ -273,6 +274,7 @@ struct esp_cmd_entry {
 	struct scsi_cmnd	*cmd;
 
 	unsigned int		saved_cur_residue;
+	struct scatterlist	*saved_prv_sg;
 	struct scatterlist	*saved_cur_sg;
 	unsigned int		saved_tot_residue;