[Qemu-devel] [PATCH 1/8] vpc: detect floppy disk geometries

[Paolo Bonzini <pbonzini@redhat.com>]

Converting a floppy image from RAW to VPC and back will generate
a zero-padded file of the wrong size, because the geometry is not
computed correctly.  Special case floppy disk images, handling
standard MS-DOS capacities (160/180/320/360 for low density
5.25" disks, 1200 for high density 5.25" disks, 720/1440/2880
for 3.5" disks).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/vpc.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/block/vpc.c b/block/vpc.c
index cb6c570..549a632 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -463,6 +463,14 @@ static int calculate_geometry(int64_t total_sectors, uint16_t* cyls,
 {
     uint32_t cyls_times_heads;
 
+    if (total_sectors <= 5760) {
+        /* Floppy disk geometry */
+        *heads = total_sectors < 640 ? 1 : 2; /* 1 = single side 5.25" */
+        *cyls = total_sectors < 1440 ? 40 : 80;  /* 40 = low density 5.25" */
+        *secs_per_cyl = total_sectors / *heads / *cyls;
+        return 0;
+    }
+
     if (total_sectors > 65535 * 16 * 255)
         return -EFBIG;
 
-- 
1.7.6