[PATCH v4 2/2] ata: pata_falcon: add data_swab option to byte-swap disk data

[Michael Schmitz <schmitzmic@gmail.com>]

Some users of pata_falcon on Q40 have IDE disks in default
IDE little endian byte order, whereas legacy disks use
host-native big-endian byte order as on the Atari Falcon.

Add module parameter 'data_swab' to allow connecting drives
with non-native data byte order. Drives selected by the
data_swap bit mask will have their user data byte-swapped to
host byte order, i.e. 'pata_falcon.data_swab=2' will byte-swap
all user data on drive B, leaving data on drive A in native
byte order. On Q40, drives on a second IDE interface may be
added to the bit mask as bits 2 and 3.

Default setting is no byte swapping, i.e. compatibility with
the native Falcon or Q40 operating system disk format.

Cc: William R Sowerbutts <will@sowerbutts.com>
Cc: Finn Thain <fthain@linux-m68k.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: William R Sowerbutts <will@sowerbutts.com>
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>

---

Changes since v2:

Geert Uytterhoeven:
- only shift swap bitmask if pdev->id > 0

Finn Thain:
- use pdev->devno directly for byte swap check

Changes since v1:

Damien Le Moal:
- change patch title
- drop swap_data flag

Finn Thain:
- drop allocation of ap->private struct, use field as bitmask

Changes since RFC v4:

Geert Uytterhoeven:
- don't shift static module parameter for drive 3/4 bitmask
- simplify bit mask calculation to always use pdev->id

Finn Thain:
- correct bit numbers for drive 3/4

Changes since RFC v3:

- split off this byte swap handling into separate patch

- add hint regarding third and fourth drive on Q40

Finn Thain:
- rename module parameter to 'data_swab' to better reflect its use

William Sowerbutts:
- correct IDE drive number used in data swap conditional
---
 drivers/ata/pata_falcon.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/ata/pata_falcon.c b/drivers/ata/pata_falcon.c
index 3841ea200bcb..7cf15bd9764a 100644
--- a/drivers/ata/pata_falcon.c
+++ b/drivers/ata/pata_falcon.c
@@ -33,6 +33,11 @@
 #define DRV_NAME "pata_falcon"
 #define DRV_VERSION "0.1.0"
 
+static int pata_falcon_swap_mask;
+
+module_param_named(data_swab, pata_falcon_swap_mask, int, 0444);
+MODULE_PARM_DESC(data_swab, "Data byte swap enable/disable bitmap (0x1==drive1, 0x2==drive2, 0x4==drive3, 0x8==drive4, default==0)");
+
 static const struct scsi_host_template pata_falcon_sht = {
 	ATA_PIO_SHT(DRV_NAME),
 };
@@ -50,7 +55,7 @@ static unsigned int pata_falcon_data_xfer(struct ata_queued_cmd *qc,
 
 	if (dev->class == ATA_DEV_ATA && cmd &&
 	    !blk_rq_is_passthrough(scsi_cmd_to_rq(cmd)))
-		swap = 0;
+		swap = (uintptr_t)ap->private_data & BIT(dev->devno);
 
 	/* Transfer multiple of 2 bytes */
 	if (rw == READ) {
@@ -194,6 +199,9 @@ static int __init pata_falcon_init_one(struct platform_device *pdev)
 	ata_port_desc(ap, "cmd %px ctl %px data %pa",
 		      base, ctl_base, &ap->ioaddr.data_addr);
 
+	ap->private_data = (void *)(uintptr_t)(pdev->id > 0 ?
+		pata_falcon_swap_mask >> 2 : pata_falcon_swap_mask);
+
 	irq_res = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
 	if (irq_res && irq_res->start > 0) {
 		irq = irq_res->start;
-- 
2.17.1