disfunctional floppy driver in kernels 4.5, 4.6 and 4.7

From: Wim Osterholt
Date: Fri Jun 10 2016 - 19:09:41 EST


up to vanilla kernel 4.4.13 floppy functionality performs like it should.
(On an x86 PC that is. With a 1.44MB diskette drive.)
>From kernel 4.5* and up it changed to barely usable.

After a virgin start (cold or warm boot) with an empty diskette drive and
then loaded with a standard 720K diskette you may run 'mdir' (from mtools)
and it shows the directory fine.
The first time you load a standard 1.44MB diskette (wether it is a virgin
start or after a 720K disktette) and you run mdir, it says literally:

plain_io: Input/output error
init A: could not read boot sector
Cannot initialize 'A:'

After this, all subsequent runs of mdir will do fine on both floppies.
However, most of my floppies are in a different format. (1.6MB)
I rely on 'setfdprm' (from fdutils) to set the correct parameters.

setfdprm /dev/fd0 1600/1440
/dev/fd0: Invalid argument

Strace shows me:
open(/dev/fd0, O_ACCMODE) = -1

So this actually means that 'invalid argument' refers to O_ACCMODE.

At all kernels I see no changed rights in:
ls -al /dev/fd0
brw-rw---- 1 root disk 2, 0 May 13 16:43 /dev/fd0

It looks to me that the code in floppy.c is quite old; no changes here.
So the bug is elsewhere in the kernel.

Could someone please explain and repair the magic that is happening here?

Thanks in advance, Wim Osterholt.

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