[PATCH 4/5] sg_ring: Convert core scsi code to sg_ring.

From: Rusty Russell
Date: Thu Dec 20 2007 - 00:52:23 EST

(Update: explicit sg member to replace use_sg indicating it's a scatter gather)

Details:
1) The use_sg (and __use_sg) fields are removed: sg_rings contain their own
count, and it's confusing to have two.
2) use_sg used to do double duty: if non-zero, it meant that the request_buffer
actually pointed to a scatterlist. Replace this with an explicit sg member:
if NULL, then request_buffer contains a pointer to the raw data.
3) The scsi_sg_count(), scsi_sglist() and scsi_bufflen() wrappers are removed.
scsi_sg_count() is no longer necessary, scsi_sglist() is now cmd->sg, and
scsi_bufflen should just be cmd->request_bufflen if you need it.

If nothing else, the simplification of this logic shows why I prefer
sg_ring over scatterlist chaining.

Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -617,20 +617,21 @@ void scsi_eh_prep_cmnd(struct scsi_cmnd
ses->cmd_len = scmd->cmd_len;
memcpy(ses->cmnd, scmd->cmnd, sizeof(scmd->cmnd));
ses->data_direction = scmd->sc_data_direction;
+ ses->sg = scmd->sg;
ses->bufflen = scmd->request_bufflen;
ses->buffer = scmd->request_buffer;
- ses->use_sg = scmd->use_sg;
ses->resid = scmd->resid;
ses->result = scmd->result;

if (sense_bytes) {
scmd->request_bufflen = min_t(unsigned,
sizeof(scmd->sense_buffer), sense_bytes);
- sg_init_one(&ses->sense_sgl, scmd->sense_buffer,
- scmd->request_bufflen);
- scmd->request_buffer = &ses->sense_sgl;
+
+ sg_ring_single(&ses->sense_sg.ring, scmd->sense_buffer,
+ scmd->request_bufflen);
+ scmd->sg = &ses->sense_sg.ring;
+ scmd->request_buffer = NULL;
scmd->sc_data_direction = DMA_FROM_DEVICE;
- scmd->use_sg = 1;
memset(scmd->cmnd, 0, sizeof(scmd->cmnd));
scmd->cmnd[0] = REQUEST_SENSE;
scmd->cmnd[4] = scmd->request_bufflen;
@@ -639,7 +640,7 @@ void scsi_eh_prep_cmnd(struct scsi_cmnd
scmd->request_buffer = NULL;
scmd->request_bufflen = 0;
scmd->sc_data_direction = DMA_NONE;
- scmd->use_sg = 0;
+ scmd->sg = NULL;
if (cmnd) {
memset(scmd->cmnd, 0, sizeof(scmd->cmnd));
memcpy(scmd->cmnd, cmnd, cmnd_size);
@@ -678,7 +679,7 @@ void scsi_eh_restore_cmnd(struct scsi_cm
scmd->sc_data_direction = ses->data_direction;
scmd->request_bufflen = ses->bufflen;
scmd->request_buffer = ses->buffer;
- scmd->use_sg = ses->use_sg;
+ scmd->sg = ses->sg;
scmd->resid = ses->resid;
scmd->result = ses->result;
}
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -17,7 +17,7 @@
#include <linux/pci.h>
#include <linux/delay.h>
#include <linux/hardirq.h>
-#include <linux/scatterlist.h>
+#include <linux/sg_ring.h>

#include <scsi/scsi.h>
#include <scsi/scsi_cmnd.h>
@@ -737,21 +737,41 @@ static inline unsigned int scsi_sgtable_
return index;
}

-struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, gfp_t gfp_mask)
+static void free_sgring(struct sg_ring *head)
{
struct scsi_host_sg_pool *sgp;
- struct scatterlist *sgl, *prev, *ret;
+ struct sg_ring *sg, *n;
+
+ /* Free any other entries in the ring. */
+ list_for_each_entry_safe(sg, n, &head->list, list) {
+ list_del(&sg->list);
+ sgp = scsi_sg_pools + scsi_sgtable_index(sg->max);
+ mempool_free(sg, sgp->pool);
+ }
+
+ /* Now free the head of the ring. */
+ BUG_ON(!list_empty(&head->list));
+
+ sgp = scsi_sg_pools + scsi_sgtable_index(head->max);
+ mempool_free(head, sgp->pool);
+}
+
+struct sg_ring *scsi_alloc_sgring(struct scsi_cmnd *cmd, unsigned int num,
+ gfp_t gfp_mask)
+{
+ struct scsi_host_sg_pool *sgp;
+ struct sg_ring *sg, *ret;
unsigned int index;
int this, left;

- BUG_ON(!cmd->use_sg);
+ BUG_ON(!num);

- left = cmd->use_sg;
- ret = prev = NULL;
+ left = num;
+ ret = NULL;
do {
this = left;
if (this > SCSI_MAX_SG_SEGMENTS) {
- this = SCSI_MAX_SG_SEGMENTS - 1;
+ this = SCSI_MAX_SG_SEGMENTS;
index = SG_MEMPOOL_NR - 1;
} else
index = scsi_sgtable_index(this);
@@ -760,32 +780,20 @@ struct scatterlist *scsi_alloc_sgtable(s

sgp = scsi_sg_pools + index;

- sgl = mempool_alloc(sgp->pool, gfp_mask);
- if (unlikely(!sgl))
+ sg = mempool_alloc(sgp->pool, gfp_mask);
+ if (unlikely(!sg))
goto enomem;

- sg_init_table(sgl, sgp->size);
+ sg_ring_init(sg, sgp->size);

/*
- * first loop through, set initial index and return value
+ * first loop through, set return value, otherwise
+ * attach this to the tail.
*/
if (!ret)
- ret = sgl;
-
- /*
- * chain previous sglist, if any. we know the previous
- * sglist must be the biggest one, or we would not have
- * ended up doing another loop.
- */
- if (prev)
- sg_chain(prev, SCSI_MAX_SG_SEGMENTS, sgl);
-
- /*
- * if we have nothing left, mark the last segment as
- * end-of-list
- */
- if (!left)
- sg_mark_end(&sgl[this - 1]);
+ ret = sg;
+ else
+ list_add_tail(&sg->list, &ret->list);

/*
* don't allow subsequent mempool allocs to sleep, it would
@@ -793,85 +801,21 @@ struct scatterlist *scsi_alloc_sgtable(s
*/
gfp_mask &= ~__GFP_WAIT;
gfp_mask |= __GFP_HIGH;
- prev = sgl;
} while (left);

- /*
- * ->use_sg may get modified after dma mapping has potentially
- * shrunk the number of segments, so keep a copy of it for free.
- */
- cmd->__use_sg = cmd->use_sg;
return ret;
enomem:
- if (ret) {
- /*
- * Free entries chained off ret. Since we were trying to
- * allocate another sglist, we know that all entries are of
- * the max size.
- */
- sgp = scsi_sg_pools + SG_MEMPOOL_NR - 1;
- prev = ret;
- ret = &ret[SCSI_MAX_SG_SEGMENTS - 1];
-
- while ((sgl = sg_chain_ptr(ret)) != NULL) {
- ret = &sgl[SCSI_MAX_SG_SEGMENTS - 1];
- mempool_free(sgl, sgp->pool);
- }
-
- mempool_free(prev, sgp->pool);
- }
+ if (ret)
+ free_sgring(ret);
return NULL;
}
+EXPORT_SYMBOL(scsi_alloc_sgring);

-EXPORT_SYMBOL(scsi_alloc_sgtable);
-
-void scsi_free_sgtable(struct scsi_cmnd *cmd)
+void scsi_free_sgring(struct scsi_cmnd *cmd)
{
- struct scatterlist *sgl = cmd->request_buffer;
- struct scsi_host_sg_pool *sgp;
-
- /*
- * if this is the biggest size sglist, check if we have
- * chained parts we need to free
- */
- if (cmd->__use_sg > SCSI_MAX_SG_SEGMENTS) {
- unsigned short this, left;
- struct scatterlist *next;
- unsigned int index;
-
- left = cmd->__use_sg - (SCSI_MAX_SG_SEGMENTS - 1);
- next = sg_chain_ptr(&sgl[SCSI_MAX_SG_SEGMENTS - 1]);
- while (left && next) {
- sgl = next;
- this = left;
- if (this > SCSI_MAX_SG_SEGMENTS) {
- this = SCSI_MAX_SG_SEGMENTS - 1;
- index = SG_MEMPOOL_NR - 1;
- } else
- index = scsi_sgtable_index(this);
-
- left -= this;
-
- sgp = scsi_sg_pools + index;
-
- if (left)
- next = sg_chain_ptr(&sgl[sgp->size - 1]);
-
- mempool_free(sgl, sgp->pool);
- }
-
- /*
- * Restore original, will be freed below
- */
- sgl = cmd->request_buffer;
- sgp = scsi_sg_pools + SG_MEMPOOL_NR - 1;
- } else
- sgp = scsi_sg_pools + scsi_sgtable_index(cmd->__use_sg);
-
- mempool_free(sgl, sgp->pool);
+ free_sgring(cmd->sg);
}
-
-EXPORT_SYMBOL(scsi_free_sgtable);
+EXPORT_SYMBOL(scsi_free_sgring);

/*
* Function: scsi_release_buffers()
@@ -892,13 +836,14 @@ EXPORT_SYMBOL(scsi_free_sgtable);
*/
static void scsi_release_buffers(struct scsi_cmnd *cmd)
{
- if (cmd->use_sg)
- scsi_free_sgtable(cmd);
+ if (cmd->sg)
+ scsi_free_sgring(cmd);

/*
* Zero these out. They now point to freed memory, and it is
* dangerous to hang onto the pointers.
*/
+ cmd->sg = NULL;
cmd->request_buffer = NULL;
cmd->request_bufflen = 0;
}
@@ -976,7 +921,10 @@ void scsi_io_completion(struct scsi_cmnd
SCSI_LOG_HLCOMPLETE(1, printk("%ld sectors total, "
"%d bytes done.\n",
req->nr_sectors, good_bytes));
- SCSI_LOG_HLCOMPLETE(1, printk("use_sg is %d\n", cmd->use_sg));
+ SCSI_LOG_HLCOMPLETE(1, printk("sg elems %d%s\n",
+ cmd->sg ? cmd->sg->num : 0,
+ cmd->sg && !list_empty(&cmd->sg->list) ?
+ "+" : ""));

if (clear_errors)
req->errors = 0;
@@ -1106,20 +1054,20 @@ static int scsi_init_io(struct scsi_cmnd
static int scsi_init_io(struct scsi_cmnd *cmd)
{
struct request *req = cmd->request;
- int count;

/*
* We used to not use scatter-gather for single segment request,
* but now we do (it makes highmem I/O easier to support without
* kmapping pages)
*/
- cmd->use_sg = req->nr_phys_segments;

+ cmd->request_buffer = NULL;
/*
* If sg table allocation fails, requeue request later.
*/
- cmd->request_buffer = scsi_alloc_sgtable(cmd, GFP_ATOMIC);
- if (unlikely(!cmd->request_buffer)) {
+ cmd->sg = scsi_alloc_sgring(cmd, req->nr_phys_segments, GFP_ATOMIC);
+ if (unlikely(!cmd->sg)) {
+ BUG();
scsi_unprep_request(req);
return BLKPREP_DEFER;
}
@@ -1134,9 +1082,7 @@ static int scsi_init_io(struct scsi_cmnd
* Next, walk the list, and fill in the addresses and sizes of
* each segment.
*/
- count = blk_rq_map_sg(req->q, req, cmd->request_buffer);
- BUG_ON(count > cmd->use_sg);
- cmd->use_sg = count;
+ blk_rq_map_sg_ring(req->q, req, cmd->sg);
return BLKPREP_OK;
}

@@ -1193,7 +1139,7 @@ int scsi_setup_blk_pc_cmnd(struct scsi_d

cmd->request_bufflen = 0;
cmd->request_buffer = NULL;
- cmd->use_sg = 0;
+ cmd->sg = NULL;
req->buffer = NULL;
}

@@ -1751,7 +1697,7 @@ int __init scsi_init_queue(void)

for (i = 0; i < SG_MEMPOOL_NR; i++) {
struct scsi_host_sg_pool *sgp = scsi_sg_pools + i;
- int size = sgp->size * sizeof(struct scatterlist);
+ int size = sizeof(struct sg_ring) + sgp->size * sizeof(struct scatterlist);

sgp->slab = kmem_cache_create(sgp->name, size, 0,
SLAB_HWCACHE_ALIGN, NULL);
diff --git a/drivers/scsi/scsi_lib_dma.c b/drivers/scsi/scsi_lib_dma.c
--- a/drivers/scsi/scsi_lib_dma.c
+++ b/drivers/scsi/scsi_lib_dma.c
@@ -15,22 +15,21 @@
* scsi_dma_map - perform DMA mapping against command's sg lists
* @cmd: scsi command
*
- * Returns the number of sg lists actually used, zero if the sg lists
- * is NULL, or -ENOMEM if the mapping failed.
+ * Returns -ENOMEM if the mapping failed, or the number of elements mapped
+ * (ie. sg_ring_num(cmd->sg)).
*/
int scsi_dma_map(struct scsi_cmnd *cmd)
{
- int nseg = 0;
+ if (cmd->sg) {
+ struct device *dev = cmd->device->host->shost_gendev.parent;
+ int err;

- if (scsi_sg_count(cmd)) {
- struct device *dev = cmd->device->host->shost_gendev.parent;
-
- nseg = dma_map_sg(dev, scsi_sglist(cmd), scsi_sg_count(cmd),
- cmd->sc_data_direction);
- if (unlikely(!nseg))
- return -ENOMEM;
+ err = dma_map_sg_ring(dev, cmd->sg, cmd->sc_data_direction);
+ if (err)
+ return err;
+ return sg_ring_num(cmd->sg);
}
- return nseg;
+ return 0;
}
EXPORT_SYMBOL(scsi_dma_map);

@@ -40,11 +39,10 @@ EXPORT_SYMBOL(scsi_dma_map);
*/
void scsi_dma_unmap(struct scsi_cmnd *cmd)
{
- if (scsi_sg_count(cmd)) {
+ if (cmd->sg) {
struct device *dev = cmd->device->host->shost_gendev.parent;

- dma_unmap_sg(dev, scsi_sglist(cmd), scsi_sg_count(cmd),
- cmd->sc_data_direction);
+ dma_unmap_sg_ring(dev, cmd->sg, cmd->sc_data_direction);
}
}
EXPORT_SYMBOL(scsi_dma_unmap);
diff --git a/drivers/scsi/scsi_tgt_lib.c b/drivers/scsi/scsi_tgt_lib.c
--- a/drivers/scsi/scsi_tgt_lib.c
+++ b/drivers/scsi/scsi_tgt_lib.c
@@ -331,7 +331,7 @@ static void scsi_tgt_cmd_done(struct scs

scsi_tgt_uspace_send_status(cmd, tcmd->itn_id, tcmd->tag);

- if (cmd->request_buffer)
+ if (cmd->sg)
scsi_free_sgtable(cmd);

queue_work(scsi_tgtd, &tcmd->work);
@@ -358,15 +358,14 @@ static int scsi_tgt_init_cmd(struct scsi
struct request *rq = cmd->request;
int count;

- cmd->use_sg = rq->nr_phys_segments;
- cmd->request_buffer = scsi_alloc_sgtable(cmd, gfp_mask);
- if (!cmd->request_buffer)
+ cmd->sg = scsi_alloc_sgring(cmd, rq->nr_phys_segments, gfp_mask);
+ if (!cmd->sg)
return -ENOMEM;
-
+ cmd->request_buffer = NULL;
cmd->request_bufflen = rq->data_len;

- dprintk("cmd %p cnt %d %lu\n", cmd, cmd->use_sg, rq_data_dir(rq));
- count = blk_rq_map_sg(rq->q, rq, cmd->request_buffer);
+ dprintk("cmd %p cnt %d %lu\n", cmd, rq->nr_phys_segments, rq_data_dir(rq));
+ count = blk_rq_map_sg_ring(rq->q, rq, cmd->request_buffer);
BUG_ON(count > cmd->use_sg);
cmd->use_sg = count;
return 0;
diff --git a/include/scsi/scsi_cmnd.h b/include/scsi/scsi_cmnd.h
--- a/include/scsi/scsi_cmnd.h
+++ b/include/scsi/scsi_cmnd.h
@@ -65,11 +65,8 @@ struct scsi_cmnd {
unsigned request_bufflen; /* Actual request size */

struct timer_list eh_timeout; /* Used to time out the command. */
- void *request_buffer; /* Actual requested buffer */
-
- /* These elements define the operation we ultimately want to perform */
- unsigned short use_sg; /* Number of pieces of scatter-gather */
- unsigned short __use_sg;
+ struct sg_ring *sg; /* if it's a scatter-gather, otherwise... */
+ void *request_buffer; /* Actual requested buffer */

unsigned underflow; /* Return error if less than
this amount is transferred */
@@ -128,15 +125,11 @@ extern void *scsi_kmap_atomic_sg(struct
size_t *offset, size_t *len);
extern void scsi_kunmap_atomic_sg(void *virt);

-extern struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *, gfp_t);
-extern void scsi_free_sgtable(struct scsi_cmnd *);
+extern struct sg_ring *scsi_alloc_sgring(struct scsi_cmnd *, unsigned, gfp_t);
+extern void scsi_free_sgring(struct scsi_cmnd *);

extern int scsi_dma_map(struct scsi_cmnd *cmd);
extern void scsi_dma_unmap(struct scsi_cmnd *cmd);
-
-#define scsi_sg_count(cmd) ((cmd)->use_sg)
-#define scsi_sglist(cmd) ((struct scatterlist *)(cmd)->request_buffer)
-#define scsi_bufflen(cmd) ((cmd)->request_bufflen)

static inline void scsi_set_resid(struct scsi_cmnd *cmd, int resid)
{
diff --git a/include/scsi/scsi_eh.h b/include/scsi/scsi_eh.h
--- a/include/scsi/scsi_eh.h
+++ b/include/scsi/scsi_eh.h
@@ -1,7 +1,7 @@
#ifndef _SCSI_SCSI_EH_H
#define _SCSI_SCSI_EH_H

-#include <linux/scatterlist.h>
+#include <linux/sg_ring.h>

#include <scsi/scsi_cmnd.h>
struct scsi_device;
@@ -75,10 +75,10 @@ struct scsi_eh_save {

void *buffer;
unsigned bufflen;
- unsigned short use_sg;
+ struct sg_ring *sg;
int resid;

- struct scatterlist sense_sgl;
+ DECLARE_SG_RING(sense_sg, 1);
};

extern void scsi_eh_prep_cmnd(struct scsi_cmnd *scmd,
--
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