[PATCH-RFC] Promise TX4 implement hw-bug workaround

[Alexander Sabourenkov <screwdriver@lxnt.info>]

Hello.
Once again,

There appears to be a hardware bug in that it chokes on scatterlist
if the last item is larger than 164 bytes. This was discovered by
reading the code of vendor-supplied driver.

The patch that follows fixes my problem on 2.6.22.

I can't think of a way to avoid second pass over scatterlist without
duplicating code (ata_qc_prep() and ata_fill_sg() from libata-core.c).





--- a/drivers/ata/sata_promise.c	2007-07-09 03:32:17.000000000 +0400
+++ b/drivers/ata/sata_promise.c	2007-10-27 19:12:46.000000000 +0400
@@ -531,6 +531,87 @@
 	memcpy(buf+31, cdb, cdb_len);
 }

+/**
+ *	pdc_fill_sg - Fill PCI IDE PRD table
+ *	@qc: Metadata associated with taskfile to be transferred
+ *
+ *	Fill PCI IDE PRD (scatter-gather) table with segments
+ *	associated with the current disk command.
+ *	Make sure hardware does not choke on it.
+ *
+ *	LOCKING:
+ *	spin_lock_irqsave(host lock)
+ *
+ */
+static void pdc_fill_sg(struct ata_queued_cmd *qc)
+{
+	struct ata_port *ap = qc->ap;
+	struct scatterlist *sg;
+	unsigned int idx;
+	const u32 SG_COUNT_ASIC_BUG = 41*4;
+
+	if (!(qc->flags & ATA_QCFLAG_DMAMAP))
+		return;
+	
+	WARN_ON(qc->__sg == NULL);
+	WARN_ON(qc->n_elem == 0 && qc->pad_len == 0);
+
+	idx = 0;
+	ata_for_each_sg(sg, qc) {
+		u32 addr, offset;
+		u32 sg_len, len;
+
+		/* determine if physical DMA addr spans 64K boundary.
+		 * Note h/w doesn't support 64-bit, so we unconditionally
+		 * truncate dma_addr_t to u32.
+		 */
+		addr = (u32) sg_dma_address(sg);
+		sg_len = sg_dma_len(sg);
+
+		while (sg_len) {
+			offset = addr & 0xffff;
+			len = sg_len;
+			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);
+
+			idx++;
+			sg_len -= len;
+			addr += len;
+		}
+	}
+
+	if (idx) {
+		u32 len = le32_to_cpu(ap->prd[idx - 1].flags_len);
+
+		if (len > SG_COUNT_ASIC_BUG) {
+			u32 addr;
+			/* if len < 2*SG_COUNT_ASIC_BUG then last
+			   segment will be larger than next-to-last.
+			   Somewhat ugly :(
+			*/
+
+			VPRINTK("Splitting last PRD.\n");
+
+			ap->prd[idx - 1].flags_len -= cpu_to_le32(SG_COUNT_ASIC_BUG);
+			VPRINTK("PRD[%u] = (0x%X, 0x%X)\n", idx - 1, addr, SG_COUNT_ASIC_BUG);
+			
+			addr = le32_to_cpu(ap->prd[idx - 1].addr) + len - SG_COUNT_ASIC_BUG;
+			len  = SG_COUNT_ASIC_BUG;
+			ap->prd[idx].addr = cpu_to_le32(addr);
+			ap->prd[idx].flags_len = cpu_to_le32(len);
+			VPRINTK("PRD[%u] = (0x%X, 0x%X)\n", idx, addr, len);
+
+			idx++;
+		}
+
+		ap->prd[idx - 1].flags_len |= cpu_to_le32(ATA_PRD_EOT);
+	}
+}
+
 static void pdc_qc_prep(struct ata_queued_cmd *qc)
 {
 	struct pdc_port_priv *pp = qc->ap->private_data;
@@ -540,7 +621,7 @@

 	switch (qc->tf.protocol) {
 	case ATA_PROT_DMA:
-		ata_qc_prep(qc);
+		pdc_fill_sg(qc);
 		/* fall through */

 	case ATA_PROT_NODATA:
@@ -556,11 +637,11 @@
 		break;

 	case ATA_PROT_ATAPI:
-		ata_qc_prep(qc);
+		pdc_fill_sg(qc);
 		break;

 	case ATA_PROT_ATAPI_DMA:
-		ata_qc_prep(qc);
+		pdc_fill_sg(qc);
 		/*FALLTHROUGH*/
 	case ATA_PROT_ATAPI_NODATA:
 		pdc_atapi_pkt(qc);