[PATCH] ppc64: Fix drivers/ide when using an IOMMU

Make IDE advertise only 128 entries of SG table on archs with an IOMMU.

The current IOMMU implementations of pci_map_sg() may produce segments
that don't match the boundary requirements of IDE, thus causing the
driver to break them up.  The BIO is supposed to account for that,
however, it cannot account for a pci_map_sg producing a segment of the
requested size, but with incorrect alignement, thus we may still try to
break up the list in more entries than is supported by the HW.

A similar fix already went in libata.  The "real" long term fix will be
to move the boundary requirements to struct device so that pci_map_sg()
can respect them when producing the sglist.  In the meantime, this
band-aid works around the problem.

drivers/ide/ide-probe.c

@@ -50,6 +50,7 @@
 #include <linux/spinlock.h>
 #include <linux/pci.h>
 #include <linux/kmod.h>
+#include <linux/pci.h>
 
 #include <asm/byteorder.h>
 #include <asm/irq.h>
@@ -904,6 +905,7 @@ static int ide_init_queue(ide_drive_t *drive)
 	request_queue_t *q;
 	ide_hwif_t *hwif = HWIF(drive);
 	int max_sectors = 256;
+	int max_sg_entries = PRD_ENTRIES;
 
 	/*
 	 *	Our default set up assumes the normal IDE case,
@@ -926,11 +928,22 @@ static int ide_init_queue(ide_drive_t *drive)
 		max_sectors = hwif->rqsize;
 	blk_queue_max_sectors(q, max_sectors);
 
-	/* IDE DMA can do PRD_ENTRIES number of segments. */
-	blk_queue_max_hw_segments(q, PRD_ENTRIES);
+#ifdef CONFIG_PCI
+	/* When we have an IOMMU, we may have a problem where pci_map_sg()
+	 * creates segments that don't completely match our boundary
+	 * requirements and thus need to be broken up again. Because it
+	 * doesn't align properly neither, we may actually have to break up
+	 * to more segments than what was we got in the first place, a max
+	 * worst case is twice as many.
+	 * This will be fixed once we teach pci_map_sg() about our boundary
+	 * requirements, hopefully soon
+	 */
+	if (!PCI_DMA_BUS_IS_PHYS)
+		max_sg_entries >>= 1;
+#endif /* CONFIG_PCI */
 
-	/* This is a driver limit and could be eliminated. */
-	blk_queue_max_phys_segments(q, PRD_ENTRIES);
+	blk_queue_max_hw_segments(q, max_sg_entries);
+	blk_queue_max_phys_segments(q, max_sg_entries);
 
 	/* assign drive and gendisk queue */
 	drive->queue = q;