block: allow disk to have extended device number

Now that disk and partition handlings are mostly unified, it's easy to
allow disk to have extended device number.  This patch makes
add_disk() use extended device number if disk->minors is zero.  Both
sd and ide-disk are updated to use this.

* sd_format_disk_name() is implemented which can generically determine
  the drive name.  This removes disk number restriction stemming from
  limited device names.

* If sd index goes over SD_MAX_DISKS (which can be increased now BTW),
  sd simply doesn't initialize minors letting block layer choose
  extended device number.

* If CONFIG_DEBUG_EXT_DEVT is set, both sd and ide-disk always set
  minors to 0 and use extended device numbers.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 54 insertions, 20 deletions

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 6598024531dd..bcb04b2a7676 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -89,7 +89,7 @@ MODULE_ALIAS_SCSI_DEVICE(TYPE_RBC);
 #if !defined(CONFIG_DEBUG_BLOCK_EXT_DEVT)
 #define SD_MINORS       16
 #else
-#define SD_MINORS       1
+#define SD_MINORS       0
 #endif
 
 static int  sd_revalidate_disk(struct gendisk *);
@@ -1770,6 +1770,52 @@ static int sd_revalidate_disk(struct gendisk *disk)
 }
 
 /**
+ *      sd_format_disk_name - format disk name
+ *      @prefix: name prefix - ie. "sd" for SCSI disks
+ *      @index: index of the disk to format name for
+ *      @buf: output buffer
+ *      @buflen: length of the output buffer
+ *
+ *      SCSI disk names starts at sda.  The 26th device is sdz and the
+ *      27th is sdaa.  The last one for two lettered suffix is sdzz
+ *      which is followed by sdaaa.
+ *
+ *      This is basically 26 base counting with one extra 'nil' entry
+ *      at the beggining from the second digit on and can be
+ *      determined using similar method as 26 base conversion with the
+ *      index shifted -1 after each digit is computed.
+ *
+ *      CONTEXT:
+ *      Don't care.
+ *
+ *      RETURNS:
+ *      0 on success, -errno on failure.
+ */
+static int sd_format_disk_name(char *prefix, int index, char *buf, int buflen)
+{
+        const int base = 'z' - 'a' + 1;
+        char *begin = buf + strlen(prefix);
+        char *end = buf + buflen;
+        char *p;
+        int unit;
+
+        p = end - 1;
+        *p = '\0';
+        unit = base;
+        do {
+                if (p == begin)
+                        return -EINVAL;
+                *--p = 'a' + (index % unit);
+                index = (index / unit) - 1;
+        } while (index >= 0);
+
+        memmove(begin, p, end - p);
+        memcpy(buf, prefix, strlen(prefix));
+
+        return 0;
+}
+
+/**
  *      sd_probe - called during driver initialization and whenever a
  *      new scsi device is attached to the system. It is called once
  *      for each scsi device (not just disks) present.
@@ -1821,8 +1867,8 @@ static int sd_probe(struct device *dev)
         if (error)
                 goto out_put;
 
-        error = -EBUSY;
-        if (index >= SD_MAX_DISKS)
+        error = sd_format_disk_name("sd", index, gd->disk_name, DISK_NAME_LEN);
+        if (error)
                 goto out_free_index;
 
         sdkp->device = sdp;
@@ -1849,24 +1895,12 @@ static int sd_probe(struct device *dev)
 
         get_device(&sdp->sdev_gendev);
 
-        gd->major = sd_major((index & 0xf0) >> 4);
-        gd->first_minor = ((index & 0xf) << 4) | (index & 0xfff00);
-        gd->minors = SD_MINORS;
-        gd->fops = &sd_fops;
-
-        if (index < 26) {
-                sprintf(gd->disk_name, "sd%c", 'a' + index % 26);
-        } else if (index < (26 + 1) * 26) {
-                sprintf(gd->disk_name, "sd%c%c",
-                        'a' + index / 26 - 1,'a' + index % 26);
-        } else {
-                const unsigned int m1 = (index / 26 - 1) / 26 - 1;
-                const unsigned int m2 = (index / 26 - 1) % 26;
-                const unsigned int m3 =  index % 26;
-                sprintf(gd->disk_name, "sd%c%c%c",
-                        'a' + m1, 'a' + m2, 'a' + m3);
+        if (index < SD_MAX_DISKS) {
+                gd->major = sd_major((index & 0xf0) >> 4);
+                gd->first_minor = ((index & 0xf) << 4) | (index & 0xfff00);
+                gd->minors = SD_MINORS;
         }
-
+        gd->fops = &sd_fops;
         gd->private_data = &sdkp->driver;
         gd->queue = sdkp->device->request_queue;