diskio: allow fallback from EDD to CHS

At least one system has been found which require fallback from EDD to
CHS, so actually make it doable.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

diff --git a/core/fs/diskio.c b/core/fs/diskio.c
index d4901c3..746335f 100644 (file)
--- a/core/fs/diskio.c
+++ b/core/fs/diskio.c
@@ -23,6 +23,10 @@ static int chs_rdwr_sectors(struct disk *disk, void *buf,
     size_t done = 0;
     size_t bytes;
     int retry;
+    uint32_t maxtransfer = disk->maxtransfer;
+
+    if (maxtransfer > 63)
+       maxtransfer = 63;
 
     memset(&ireg, 0, sizeof ireg);
 
@@ -31,8 +35,8 @@ static int chs_rdwr_sectors(struct disk *disk, void *buf,
 
     while (count) {
        chunk = count;
-       if (chunk > disk->maxtransfer)
-           chunk = disk->maxtransfer;
+       if (chunk > maxtransfer)
+           chunk = maxtransfer;
 
        freeseg = (0x10000 - ((size_t)ptr & 0xffff)) >> sector_shift;
 
@@ -47,16 +51,19 @@ static int chs_rdwr_sectors(struct disk *disk, void *buf,
        if (chunk > freeseg)
            chunk = freeseg;
 
-       bytes = chunk << sector_shift;
-
-       if (tptr != ptr && is_write)
-           memcpy(tptr, ptr, bytes);
-
        s = xlba % disk->s;
        t = xlba / disk->s;
        h = t % disk->h;
        c = t / disk->h;
 
+       if (chunk > (disk->s - s))
+           chunk = disk->s - s;
+
+       bytes = chunk << sector_shift;
+
+       if (tptr != ptr && is_write)
+           memcpy(tptr, ptr, bytes);
+
        ireg.eax.b[0] = chunk;
        ireg.ecx.b[1] = c;
        ireg.ecx.b[0] = ((c & 0x300) >> 2) | (s+1);
@@ -67,34 +74,39 @@ static int chs_rdwr_sectors(struct disk *disk, void *buf,
        retry = RETRY_COUNT;
 
         for (;;) {
-           dprintf("CHS[%02x]: %u @ %llu (%u/%u/%u) %04x:%04x %s %p\n",
-                   ireg.edx.b[0], chunk, xlba, c, h, s+1,
-                   ireg.es, ireg.ebx.w[0],
-                   (ireg.eax.b[1] & 1) ? "<-" : "->",
-                   ptr);
-
-           __intcall(0x13, &ireg, &oreg);
-           if (!(oreg.eflags.l & EFLAGS_CF))
-               break;
-
-           dprintf("CHS: error AX = %04x\n", oreg.eax.w[0]);
-
-           if (retry--)
-               continue;
-
-           /* For any starting value, this will always end with ..., 1, 0 */
-           chunk >>= 1;
-            if (chunk) {
-               disk->maxtransfer = chunk;
-               retry = RETRY_COUNT;
-                ireg.eax.b[0] = chunk;
-                continue;
-           } else {
-               printf("CHS: Error %04x %s sector %llu (%u/%u/%u)\n",
-                      oreg.eax.w[0],
-                      is_write ? "writing" : "reading",
-                      lba, c, h, s+1);
+           if (c < 1024) {
+               dprintf("CHS[%02x]: %u @ %llu (%u/%u/%u) %04x:%04x %s %p\n",
+                       ireg.edx.b[0], chunk, xlba, c, h, s+1,
+                       ireg.es, ireg.ebx.w[0],
+                       (ireg.eax.b[1] & 1) ? "<-" : "->",
+                       ptr);
+
+               __intcall(0x13, &ireg, &oreg);
+               if (!(oreg.eflags.l & EFLAGS_CF))
+                   break;
+
+               dprintf("CHS: error AX = %04x\n", oreg.eax.w[0]);
+
+               if (retry--)
+                   continue;
+
+               /*
+                * For any starting value, this will always end with
+                * ..., 1, 0
+                */
+               chunk >>= 1;
+               if (chunk) {
+                   maxtransfer = chunk;
+                   retry = RETRY_COUNT;
+                   ireg.eax.b[0] = chunk;
+                   continue;
+               }
            }
+
+           printf("CHS: Error %04x %s sector %llu (%u/%u/%u)\n",
+                  oreg.eax.w[0],
+                  is_write ? "writing" : "reading",
+                  lba, c, h, s+1);
            return done;        /* Failure */
        }
 
@@ -103,6 +115,9 @@ static int chs_rdwr_sectors(struct disk *disk, void *buf,
        if (tptr != ptr && !is_write)
            memcpy(ptr, tptr, bytes);
 
+       /* If we dropped maxtransfer, it eventually worked, so remember it */
+       disk->maxtransfer = maxtransfer;
+
        ptr   += bytes;
        xlba  += chunk;
        count -= chunk;
@@ -131,6 +146,7 @@ static int edd_rdwr_sectors(struct disk *disk, void *buf,
     size_t done = 0;
     size_t bytes;
     int retry;
+    uint32_t maxtransfer = disk->maxtransfer;
 
     memset(&ireg, 0, sizeof ireg);
 
@@ -146,8 +162,8 @@ static int edd_rdwr_sectors(struct disk *disk, void *buf,
     lba += disk->part_start;
     while (count) {
        chunk = count;
-       if (chunk > disk->maxtransfer)
-           chunk = disk->maxtransfer;
+       if (chunk > maxtransfer)
+           chunk = maxtransfer;
 
        freeseg = (0x10000 - ((size_t)ptr & 0xffff)) >> sector_shift;
 
@@ -201,13 +217,26 @@ static int edd_rdwr_sectors(struct disk *disk, void *buf,
            /* For any starting value, this will always end with ..., 1, 0 */
            chunk >>= 1;
            if (chunk) {
-               disk->maxtransfer = chunk;
+               maxtransfer = chunk;
                retry = RETRY_COUNT;
                continue;
            }
 
-           /*** XXX: Consider falling back to CHS here?! ***/
-            printf("EDD: Error %04x %s sector %llu\n",
+           /*
+            * Total failure.  There are systems which identify as
+            * EDD-capable but aren't; the known such systems return
+            * error code AH=1 (invalid function), but let's not
+            * assume that for now.
+            */
+           if (lba < ((disk->h * disk->s) << 10)) {
+               done = chs_rdwr_sectors(disk, buf, lba, count, is_write);
+               if (done == (count << sector_shift)) {
+                   /* Successful, assume this is a CHS disk */
+                   disk->rdwr_sectors = chs_rdwr_sectors;
+                   return done;
+               }
+           }
+           printf("EDD: Error %04x %s sector %llu\n",
                   oreg.eax.w[0],
                   is_write ? "writing" : "reading",
                   lba);
@@ -219,6 +248,9 @@ static int edd_rdwr_sectors(struct disk *disk, void *buf,
        if (tptr != ptr && !is_write)
            memcpy(ptr, tptr, bytes);
 
+       /* If we dropped maxtransfer, it eventually worked, so remember it */
+       disk->maxtransfer = maxtransfer;
+
        ptr   += bytes;
        lba   += chunk;
        count -= chunk;
@@ -226,6 +258,7 @@ static int edd_rdwr_sectors(struct disk *disk, void *buf,
     }
     return done;
 }
+
 struct edd_disk_params {
     uint16_t  len;
     uint16_t  flags;