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disk/format: choose default cluster size if not given

--- a/sys/src/cmd/disk/format.c

+++ b/sys/src/cmd/disk/format.c

@@ -364,6 +364,23 @@

 	return tot;

 }

+int

+defcluster(vlong n)

+{

+	int i;

+

+	i = n / 32768;

+	if(i <= 1)

+		return 1;

+	if(i >= 128)

+		return 128;

+	i--;

+	i |= i >> 1;

+	i |= i >> 2;

+	i |= i >> 4;

+	return i+1;

+}

+

 void

 dosfs(int dofat, int dopbs, Disk *disk, char *label, int argc, char *argv[], int commit)

 {

@@ -388,6 +405,7 @@

 		t->sectors = disk->s;

 		t->heads = disk->h;

 		t->tracks = disk->c;

+		t->cluster = defcluster(disk->secs);

 	}

 	if(t->sectors == 0 && dofat)

@@ -490,6 +508,8 @@

 		if(clustersize == 0)

 			clustersize = t->cluster;

+if(chatty) print("clustersize %d\n", clustersize);

+

 		/*

 		 * the number of fat bits depends on how much disk is left

 		 * over after you subtract out the space taken up by the fat tables. 

@@ -519,7 +539,8 @@

 				break;

 			clusters = newclusters;

 			if(i > 10)

-				fatal("can't decide how many clusters to use (%d? %d?)", clusters, newclusters);

+				fatal("can't decide how many clusters to use (%d? %d?)",

+					clusters, newclusters);

 if(chatty) print("clusters %d\n", clusters);

 		}

@@ -786,7 +807,7 @@

 	d = (Dosdir*)entry;

 	putname(s, d);

-	if(strcmp(s, "9load") == 0)

+	if(cistrcmp(s, "9load") == 0 || cistrncmp(s, "9boot", 5) == 0)

 		d->attr = DSYSTEM;

 	else

 		d->attr = 0;