The image size indicates it's an 81 track floppy disk image, which we
don't have a listing for in the geometry table. When you force the drive
type to 1.44MB, it guesses the reasonably close 18/80. When the drive
type is allowed to auto-detect or set to 2.88, it guesses a very
incorrect geometry.

auto, 144 and 288 drive types get the right geometry with the new entry
in the table.

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: John Snow <jsnow@redhat.com>
---
 hw/block/fdc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index 198940e737..b2f26ba587 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -122,6 +122,7 @@ static const FDFormat fd_formats[] = {
     /* First entry is default format */
     /* 1.44 MB 3"1/2 floppy disks */
     { FLOPPY_DRIVE_TYPE_144, 18, 80, 1, FDRIVE_RATE_500K, }, /* 3.5" 2880 */
+    { FLOPPY_DRIVE_TYPE_144, 18, 81, 1, FDRIVE_RATE_500K, },
     { FLOPPY_DRIVE_TYPE_144, 20, 80, 1, FDRIVE_RATE_500K, }, /* 3.5" 3200 */
     { FLOPPY_DRIVE_TYPE_144, 21, 80, 1, FDRIVE_RATE_500K, },
     { FLOPPY_DRIVE_TYPE_144, 21, 82, 1, FDRIVE_RATE_500K, },
-- 
2.29.2

On 12/03/2021 07.32, John Snow wrote:
> The image size indicates it's an 81 track floppy disk image, which we
> don't have a listing for in the geometry table. When you force the drive
> type to 1.44MB, it guesses the reasonably close 18/80. When the drive
> type is allowed to auto-detect or set to 2.88, it guesses a very
> incorrect geometry.
> 
> auto, 144 and 288 drive types get the right geometry with the new entry
> in the table.
> 
> Reported-by: Michael Tokarev <mjt@tls.msk.ru>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>   hw/block/fdc.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/hw/block/fdc.c b/hw/block/fdc.c
> index 198940e737..b2f26ba587 100644
> --- a/hw/block/fdc.c
> +++ b/hw/block/fdc.c
> @@ -122,6 +122,7 @@ static const FDFormat fd_formats[] = {
>       /* First entry is default format */
>       /* 1.44 MB 3"1/2 floppy disks */
>       { FLOPPY_DRIVE_TYPE_144, 18, 80, 1, FDRIVE_RATE_500K, }, /* 3.5" 2880 */
> +    { FLOPPY_DRIVE_TYPE_144, 18, 81, 1, FDRIVE_RATE_500K, },
>       { FLOPPY_DRIVE_TYPE_144, 20, 80, 1, FDRIVE_RATE_500K, }, /* 3.5" 3200 */
>       { FLOPPY_DRIVE_TYPE_144, 21, 80, 1, FDRIVE_RATE_500K, },
>       { FLOPPY_DRIVE_TYPE_144, 21, 82, 1, FDRIVE_RATE_500K, },

That whole table-based approach seems quite unreliable to me - I've seen 
floppy disks with 80, 81, 82 or sometimes even 83 tracks in the past, so I 
think we would do better with a more flexible way of guessing ... but for 
the time being, this is certainly a quick and easy fix that also should not 
have any negative impact, thus:

Reviewed-by: Thomas Huth <thuth@redhat.com>

On 3/12/21 3:01 AM, Thomas Huth wrote:
> 
> That whole table-based approach seems quite unreliable to me - I've seen 
> floppy disks with 80, 81, 82 or sometimes even 83 tracks in the past, so 
> I think we would do better with a more flexible way of guessing ... but 
> for the time being, this is certainly a quick and easy fix that also 
> should not have any negative impact, thus:
> 
> Reviewed-by: Thomas Huth <thuth@redhat.com>

Yes, that's my thought. I can't personally justify tackling this hokey 
system -- I simply don't know enough about the history of the device to 
design a suitable replacement.

So, it's a band-aid, but one that fits with the way the floppy has 
worked for quite a long time, and seems the least likely to bother 
anything further.

--js