ide: remove magic constants from the device register

(In QEMU, we call this the "select" register.)

My memory isn't good enough to memorize what these magic runes
do. Label them to prevent mixups from happening in the future.

Side note: I assume it's safe to always set 0xA0 even though ATA2 claims
these bits are reserved, because ATA3 immediately reinstated that these
bits should be always on. ATA4 and subsequent specs only claim that the
fields are obsolete, so I assume it's safe to leave these set and that
it should work with the widest array of guests.

Signed-off-by: John Snow <jsnow@redhat.com>

hw/ide/core.c

@@ -367,7 +367,7 @@ fill_buffer:
 
 static void ide_set_signature(IDEState *s)
 {
-    s->select &= 0xf0; /* clear head */
+    s->select &= ~(ATA_DEV_HS); /* clear head */
     /* put signature */
     s->nsector = 1;
     s->sector = 1;
@@ -586,7 +586,7 @@ void ide_transfer_stop(IDEState *s)
 int64_t ide_get_sector(IDEState *s)
 {
     int64_t sector_num;
-    if (s->select & 0x40) {
+    if (s->select & (ATA_DEV_LBA)) {
         if (s->lba48) {
             sector_num = ((int64_t)s->hob_hcyl << 40) |
                 ((int64_t) s->hob_lcyl << 32) |
@@ -595,13 +595,13 @@ int64_t ide_get_sector(IDEState *s)
                 ((int64_t) s->lcyl << 8) | s->sector;
         } else {
             /* LBA28 */
-            sector_num = ((s->select & 0x0f) << 24) | (s->hcyl << 16) |
-                (s->lcyl << 8) | s->sector;
+            sector_num = ((s->select & (ATA_DEV_LBA_MSB)) << 24) |
+                (s->hcyl << 16) | (s->lcyl << 8) | s->sector;
         }
     } else {
         /* CHS */
         sector_num = ((s->hcyl << 8) | s->lcyl) * s->heads * s->sectors +
-            (s->select & 0x0f) * s->sectors + (s->sector - 1);
+            (s->select & (ATA_DEV_HS)) * s->sectors + (s->sector - 1);
     }
 
     return sector_num;
@@ -610,7 +610,7 @@ int64_t ide_get_sector(IDEState *s)
 void ide_set_sector(IDEState *s, int64_t sector_num)
 {
     unsigned int cyl, r;
-    if (s->select & 0x40) {
+    if (s->select & (ATA_DEV_LBA)) {
         if (s->lba48) {
             s->sector = sector_num;
             s->lcyl = sector_num >> 8;
@@ -620,7 +620,8 @@ void ide_set_sector(IDEState *s, int64_t sector_num)
             s->hob_hcyl = sector_num >> 40;
         } else {
             /* LBA28 */
-            s->select = (s->select & 0xf0) | (sector_num >> 24);
+            s->select = (s->select & ~(ATA_DEV_LBA_MSB)) |
+                ((sector_num >> 24) & (ATA_DEV_LBA_MSB));
             s->hcyl = (sector_num >> 16);
             s->lcyl = (sector_num >> 8);
             s->sector = (sector_num);
@@ -631,7 +632,8 @@ void ide_set_sector(IDEState *s, int64_t sector_num)
         r = sector_num % (s->heads * s->sectors);
         s->hcyl = cyl >> 8;
         s->lcyl = cyl;
-        s->select = (s->select & 0xf0) | ((r / s->sectors) & 0x0f);
+        s->select = (s->select & ~(ATA_DEV_HS)) |
+            ((r / s->sectors) & (ATA_DEV_HS));
         s->sector = (r % s->sectors) + 1;
     }
 }
@@ -1302,10 +1304,10 @@ void ide_ioport_write(void *opaque, uint32_t addr, uint32_t val)
         break;
     case ATA_IOPORT_WR_DEVICE_HEAD:
         ide_clear_hob(bus);
-        bus->ifs[0].select = val | 0xa0;
-        bus->ifs[1].select = val | 0xa0;
+        bus->ifs[0].select = val | (ATA_DEV_ALWAYS_ON);
+        bus->ifs[1].select = val | (ATA_DEV_ALWAYS_ON);
         /* select drive */
-        bus->unit = (val >> 4) & 1;
+        bus->unit = (val & (ATA_DEV_SELECT)) ? 1 : 0;
         break;
     default:
     case ATA_IOPORT_WR_COMMAND:
@@ -1343,7 +1345,7 @@ static void ide_reset(IDEState *s)
     s->hob_lcyl = 0;
     s->hob_hcyl = 0;
 
-    s->select = 0xa0;
+    s->select = (ATA_DEV_ALWAYS_ON);
     s->status = READY_STAT | SEEK_STAT;
 
     s->lba48 = 0;

include/hw/ide/internal.h

@@ -29,6 +29,17 @@ OBJECT_DECLARE_SIMPLE_TYPE(IDEBus, IDE_BUS)
 
 #define MAX_IDE_DEVS 2
 
+/* Device/Head ("select") Register */
+#define ATA_DEV_SELECT          0x10
+/* ATA1,3: Defined as '1'.
+ * ATA2:   Reserved.
+ * ATA3-7: obsolete. */
+#define ATA_DEV_ALWAYS_ON       0xA0
+#define ATA_DEV_LBA             0x40
+#define ATA_DEV_LBA_MSB         0x0F  /* LBA 24:27 */
+#define ATA_DEV_HS              0x0F  /* HS 3:0 */
+
+
 /* Bits of HD_STATUS */
 #define ERR_STAT		0x01
 #define INDEX_STAT		0x02