libata: convert to chained sg

libata used private sg iterator to handle padding sg.  Now that sg can
be chained, padding can be handled using standard sg ops.  Convert to
chained sg.

* s/qc->__sg/qc->sg/

* s/qc->pad_sgent/qc->extra_sg[]/.  Because chaining consumes one sg
  entry.  There need to be two extra sg entries.  The renaming is also
  for future addition of other extra sg entries.

* Padding setup is moved into ata_sg_setup_extra() which is organized
  in a way that future addition of other extra sg entries is easy.

* qc->orig_n_elem is unused and removed.

* qc->n_elem now contains the number of sg entries that LLDs should
  map.  qc->mapped_n_elem is added to carry the original number of
  mapped sgs for unmapping.

* The last sg of the original sg list is used to chain to extra sg
  list.  The original last sg is pointed to by qc->last_sg and the
  content is stored in qc->saved_last_sg.  It's restored during
  ata_sg_clean().

* All sg walking code has been updated.  Unnecessary assertions and
  checks for conditions the core layer already guarantees are removed.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/ahci.c

@@ -1483,28 +1483,24 @@ static void ahci_tf_read(struct ata_port *ap, struct ata_taskfile *tf)
 static unsigned int ahci_fill_sg(struct ata_queued_cmd *qc, void *cmd_tbl)
 {
 	struct scatterlist *sg;
-	struct ahci_sg *ahci_sg;
-	unsigned int n_sg = 0;
+	struct ahci_sg *ahci_sg = cmd_tbl + AHCI_CMD_TBL_HDR_SZ;
+	unsigned int si;
 
 	VPRINTK("ENTER\n");
 
 	/*
 	 * Next, the S/G list.
 	 */
-	ahci_sg = cmd_tbl + AHCI_CMD_TBL_HDR_SZ;
-	ata_for_each_sg(sg, qc) {
+	for_each_sg(qc->sg, sg, qc->n_elem, si) {
 		dma_addr_t addr = sg_dma_address(sg);
 		u32 sg_len = sg_dma_len(sg);
 
-		ahci_sg->addr = cpu_to_le32(addr & 0xffffffff);
-		ahci_sg->addr_hi = cpu_to_le32((addr >> 16) >> 16);
-		ahci_sg->flags_size = cpu_to_le32(sg_len - 1);
-
-		ahci_sg++;
-		n_sg++;
+		ahci_sg[si].addr = cpu_to_le32(addr & 0xffffffff);
+		ahci_sg[si].addr_hi = cpu_to_le32((addr >> 16) >> 16);
+		ahci_sg[si].flags_size = cpu_to_le32(sg_len - 1);
 	}
 
-	return n_sg;
+	return si;
 }
 
 static void ahci_qc_prep(struct ata_queued_cmd *qc)

drivers/ata/libata-core.c

@@ -4471,13 +4471,13 @@ static unsigned int ata_dev_init_params(struct ata_device *dev,
 void ata_sg_clean(struct ata_queued_cmd *qc)
 {
 	struct ata_port *ap = qc->ap;
-	struct scatterlist *sg = qc->__sg;
+	struct scatterlist *sg = qc->sg;
 	int dir = qc->dma_dir;
 	void *pad_buf = NULL;
 
 	WARN_ON(sg == NULL);
 
-	VPRINTK("unmapping %u sg elements\n", qc->n_elem);
+	VPRINTK("unmapping %u sg elements\n", qc->mapped_n_elem);
 
 	/* if we padded the buffer out to 32-bit bound, and data
 	 * xfer direction is from-device, we must copy from the
@@ -4486,19 +4486,20 @@ void ata_sg_clean(struct ata_queued_cmd *qc)
 	if (qc->pad_len && !(qc->tf.flags & ATA_TFLAG_WRITE))
 		pad_buf = ap->pad + (qc->tag * ATA_DMA_PAD_SZ);
 
-	if (qc->n_elem)
-		dma_unmap_sg(ap->dev, sg, qc->n_elem, dir);
+	if (qc->mapped_n_elem)
+		dma_unmap_sg(ap->dev, sg, qc->mapped_n_elem, dir);
 	/* restore last sg */
-	sg_last(sg, qc->orig_n_elem)->length += qc->pad_len;
+	if (qc->last_sg)
+		*qc->last_sg = qc->saved_last_sg;
 	if (pad_buf) {
-		struct scatterlist *psg = &qc->pad_sgent;
+		struct scatterlist *psg = &qc->extra_sg[1];
 		void *addr = kmap_atomic(sg_page(psg), KM_IRQ0);
 		memcpy(addr + psg->offset, pad_buf, qc->pad_len);
 		kunmap_atomic(addr, KM_IRQ0);
 	}
 
 	qc->flags &= ~ATA_QCFLAG_DMAMAP;
-	qc->__sg = NULL;
+	qc->sg = NULL;
 }
 
 /**
@@ -4516,13 +4517,10 @@ static void ata_fill_sg(struct ata_queued_cmd *qc)
 {
 	struct ata_port *ap = qc->ap;
 	struct scatterlist *sg;
-	unsigned int idx;
+	unsigned int si, pi;
 
-	WARN_ON(qc->__sg == NULL);
-	WARN_ON(qc->n_elem == 0 && qc->pad_len == 0);
-
-	idx = 0;
-	ata_for_each_sg(sg, qc) {
+	pi = 0;
+	for_each_sg(qc->sg, sg, qc->n_elem, si) {
 		u32 addr, offset;
 		u32 sg_len, len;
 
@@ -4539,18 +4537,17 @@ static void ata_fill_sg(struct ata_queued_cmd *qc)
 			if ((offset + sg_len) > 0x10000)
 				len = 0x10000 - offset;
 
-			ap->prd[idx].addr = cpu_to_le32(addr);
-			ap->prd[idx].flags_len = cpu_to_le32(len & 0xffff);
-			VPRINTK("PRD[%u] = (0x%X, 0x%X)\n", idx, addr, len);
+			ap->prd[pi].addr = cpu_to_le32(addr);
+			ap->prd[pi].flags_len = cpu_to_le32(len & 0xffff);
+			VPRINTK("PRD[%u] = (0x%X, 0x%X)\n", pi, addr, len);
 
-			idx++;
+			pi++;
 			sg_len -= len;
 			addr += len;
 		}
 	}
 
-	if (idx)
-		ap->prd[idx - 1].flags_len |= cpu_to_le32(ATA_PRD_EOT);
+	ap->prd[pi - 1].flags_len |= cpu_to_le32(ATA_PRD_EOT);
 }
 
 /**
@@ -4570,13 +4567,10 @@ static void ata_fill_sg_dumb(struct ata_queued_cmd *qc)
 {
 	struct ata_port *ap = qc->ap;
 	struct scatterlist *sg;
-	unsigned int idx;
-
-	WARN_ON(qc->__sg == NULL);
-	WARN_ON(qc->n_elem == 0 && qc->pad_len == 0);
+	unsigned int si, pi;
 
-	idx = 0;
-	ata_for_each_sg(sg, qc) {
+	pi = 0;
+	for_each_sg(qc->sg, sg, qc->n_elem, si) {
 		u32 addr, offset;
 		u32 sg_len, len, blen;
 
@@ -4594,25 +4588,24 @@ static void ata_fill_sg_dumb(struct ata_queued_cmd *qc)
 				len = 0x10000 - offset;
 
 			blen = len & 0xffff;
-			ap->prd[idx].addr = cpu_to_le32(addr);
+			ap->prd[pi].addr = cpu_to_le32(addr);
 			if (blen == 0) {
 			   /* Some PATA chipsets like the CS5530 can't
 			      cope with 0x0000 meaning 64K as the spec says */
-				ap->prd[idx].flags_len = cpu_to_le32(0x8000);
+				ap->prd[pi].flags_len = cpu_to_le32(0x8000);
 				blen = 0x8000;
-				ap->prd[++idx].addr = cpu_to_le32(addr + 0x8000);
+				ap->prd[++pi].addr = cpu_to_le32(addr + 0x8000);
 			}
-			ap->prd[idx].flags_len = cpu_to_le32(blen);
-			VPRINTK("PRD[%u] = (0x%X, 0x%X)\n", idx, addr, len);
+			ap->prd[pi].flags_len = cpu_to_le32(blen);
+			VPRINTK("PRD[%u] = (0x%X, 0x%X)\n", pi, addr, len);
 
-			idx++;
+			pi++;
 			sg_len -= len;
 			addr += len;
 		}
 	}
 
-	if (idx)
-		ap->prd[idx - 1].flags_len |= cpu_to_le32(ATA_PRD_EOT);
+	ap->prd[pi - 1].flags_len |= cpu_to_le32(ATA_PRD_EOT);
 }
 
 /**
@@ -4764,54 +4757,48 @@ void ata_noop_qc_prep(struct ata_queued_cmd *qc) { }
 void ata_sg_init(struct ata_queued_cmd *qc, struct scatterlist *sg,
 		 unsigned int n_elem)
 {
-	qc->__sg = sg;
+	qc->sg = sg;
 	qc->n_elem = n_elem;
-	qc->orig_n_elem = n_elem;
-	qc->cursg = qc->__sg;
+	qc->cursg = qc->sg;
 }
 
-/**
- *	ata_sg_setup - DMA-map the scatter-gather table associated with a command.
- *	@qc: Command with scatter-gather table to be mapped.
- *
- *	DMA-map the scatter-gather table associated with queued_cmd @qc.
- *
- *	LOCKING:
- *	spin_lock_irqsave(host lock)
- *
- *	RETURNS:
- *	Zero on success, negative on error.
- *
- */
-
-static int ata_sg_setup(struct ata_queued_cmd *qc)
+static unsigned int ata_sg_setup_extra(struct ata_queued_cmd *qc,
+				       unsigned int *n_elem_extra)
 {
 	struct ata_port *ap = qc->ap;
-	struct scatterlist *sg = qc->__sg;
-	struct scatterlist *lsg = sg_last(qc->__sg, qc->n_elem);
-	int n_elem, pre_n_elem, dir, trim_sg = 0;
+	unsigned int n_elem = qc->n_elem;
+	struct scatterlist *lsg, *copy_lsg = NULL, *tsg = NULL, *esg = NULL;
 
-	VPRINTK("ENTER, ata%u\n", ap->print_id);
+	*n_elem_extra = 0;
+
+	/* needs padding? */
+	qc->pad_len = qc->nbytes & 3;
+
+	if (likely(!qc->pad_len))
+		return n_elem;
+
+	/* locate last sg and save it */
+	lsg = sg_last(qc->sg, n_elem);
+	qc->last_sg = lsg;
+	qc->saved_last_sg = *lsg;
+
+	sg_init_table(qc->extra_sg, ARRAY_SIZE(qc->extra_sg));
 
-	/* we must lengthen transfers to end on a 32-bit boundary */
-	qc->pad_len = lsg->length & 3;
 	if (qc->pad_len) {
+		struct scatterlist *psg = &qc->extra_sg[1];
 		void *pad_buf = ap->pad + (qc->tag * ATA_DMA_PAD_SZ);
-		struct scatterlist *psg = &qc->pad_sgent;
 		unsigned int offset;
 
 		WARN_ON(qc->dev->class != ATA_DEV_ATAPI);
 
 		memset(pad_buf, 0, ATA_DMA_PAD_SZ);
 
-		/*
-		 * psg->page/offset are used to copy to-be-written
+		/* psg->page/offset are used to copy to-be-written
 		 * data in this function or read data in ata_sg_clean.
 		 */
 		offset = lsg->offset + lsg->length - qc->pad_len;
-		sg_init_table(psg, 1);
 		sg_set_page(psg, nth_page(sg_page(lsg), offset >> PAGE_SHIFT),
-				qc->pad_len, offset_in_page(offset));
+			    qc->pad_len, offset_in_page(offset));
 
 		if (qc->tf.flags & ATA_TFLAG_WRITE) {
 			void *addr = kmap_atomic(sg_page(psg), KM_IRQ0);
@@ -4821,36 +4808,84 @@ static int ata_sg_setup(struct ata_queued_cmd *qc)
 
 		sg_dma_address(psg) = ap->pad_dma + (qc->tag * ATA_DMA_PAD_SZ);
 		sg_dma_len(psg) = ATA_DMA_PAD_SZ;
-		/* trim last sg */
+
+		/* Trim the last sg entry and chain the original and
+		 * padding sg lists.
+		 *
+		 * Because chaining consumes one sg entry, one extra
+		 * sg entry is allocated and the last sg entry is
+		 * copied to it if the length isn't zero after padded
+		 * amount is removed.
+		 *
+		 * If the last sg entry is completely replaced by
+		 * padding sg entry, the first sg entry is skipped
+		 * while chaining.
+		 */
 		lsg->length -= qc->pad_len;
-		if (lsg->length == 0)
-			trim_sg = 1;
+		if (lsg->length) {
+			copy_lsg = &qc->extra_sg[0];
+			tsg = &qc->extra_sg[0];
+		} else {
+			n_elem--;
+			tsg = &qc->extra_sg[1];
+		}
+
+		esg = &qc->extra_sg[1];
 
-		DPRINTK("padding done, sg[%d].length=%u pad_len=%u\n",
-			qc->n_elem - 1, lsg->length, qc->pad_len);
+		(*n_elem_extra)++;
 	}
 
-	pre_n_elem = qc->n_elem;
-	if (trim_sg && pre_n_elem)
-		pre_n_elem--;
+	if (copy_lsg)
+		sg_set_page(copy_lsg, sg_page(lsg), lsg->length, lsg->offset);
 
-	if (!pre_n_elem) {
-		n_elem = 0;
-		goto skip_map;
+	sg_chain(lsg, 1, tsg);
+	sg_mark_end(esg);
+
+	/* sglist can't start with chaining sg entry, fast forward */
+	if (qc->sg == lsg) {
+		qc->sg = tsg;
+		qc->cursg = tsg;
 	}
 
-	dir = qc->dma_dir;
-	n_elem = dma_map_sg(ap->dev, sg, pre_n_elem, dir);
-	if (n_elem < 1) {
-		/* restore last sg */
-		lsg->length += qc->pad_len;
-		return -1;
+	return n_elem;
+}
+
+/**
+ *	ata_sg_setup - DMA-map the scatter-gather table associated with a command.
+ *	@qc: Command with scatter-gather table to be mapped.
+ *
+ *	DMA-map the scatter-gather table associated with queued_cmd @qc.
+ *
+ *	LOCKING:
+ *	spin_lock_irqsave(host lock)
+ *
+ *	RETURNS:
+ *	Zero on success, negative on error.
+ *
+ */
+static int ata_sg_setup(struct ata_queued_cmd *qc)
+{
+	struct ata_port *ap = qc->ap;
+	unsigned int n_elem, n_elem_extra;
+
+	VPRINTK("ENTER, ata%u\n", ap->print_id);
+
+	n_elem = ata_sg_setup_extra(qc, &n_elem_extra);
+
+	if (n_elem) {
+		n_elem = dma_map_sg(ap->dev, qc->sg, n_elem, qc->dma_dir);
+		if (n_elem < 1) {
+			/* restore last sg */
+			if (qc->last_sg)
+				*qc->last_sg = qc->saved_last_sg;
+			return -1;
+		}
+		DPRINTK("%d sg elements mapped\n", n_elem);
 	}
 
-	DPRINTK("%d sg elements mapped\n", n_elem);
+	qc->n_elem = qc->mapped_n_elem = n_elem;
+	qc->n_elem += n_elem_extra;
 
-skip_map:
-	qc->n_elem = n_elem;
 	qc->flags |= ATA_QCFLAG_DMAMAP;
 
 	return 0;
@@ -5912,7 +5947,7 @@ void ata_qc_issue(struct ata_queued_cmd *qc)
 	/* We guarantee to LLDs that they will have at least one
 	 * non-zero sg if the command is a data command.
 	 */
-	BUG_ON(ata_is_data(prot) && (!qc->__sg || !qc->n_elem || !qc->nbytes));
+	BUG_ON(ata_is_data(prot) && (!qc->sg || !qc->n_elem || !qc->nbytes));
 
 	if (ata_is_dma(prot) || (ata_is_pio(prot) &&
 				 (ap->flags & ATA_FLAG_PIO_DMA)))

drivers/ata/libata-scsi.c

@@ -517,7 +517,7 @@ static struct ata_queued_cmd *ata_scsi_qc_new(struct ata_device *dev,
 		qc->scsicmd = cmd;
 		qc->scsidone = done;
 
-		qc->__sg = scsi_sglist(cmd);
+		qc->sg = scsi_sglist(cmd);
 		qc->n_elem = scsi_sg_count(cmd);
 	} else {
 		cmd->result = (DID_OK << 16) | (QUEUE_FULL << 1);