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Re: [Qemu-devel] [PATCH] Fix IDE FDC emulation for no media

From: Pavel Hrdina
Subject: Re: [Qemu-devel] [PATCH] Fix IDE FDC emulation for no media
Date: Tue, 24 Apr 2012 12:28:16 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1 


On 04/24/2012 12:23 PM, Kevin Wolf wrote:

Am 24.04.2012 11:55, schrieb Pavel Hrdina:

On 04/24/2012 11:32 AM, Kevin Wolf wrote:

Am 23.04.2012 18:06, schrieb Pavel Hrdina:

Hi,
this is the patch to fix incorrect handling of IDE floppy drive controller 
emulation
when no media is present. If the guest is booted without a media then the drive
was not being emulated at all but this patch enables the emulation with no 
media present.

There was a bug in FDC emulation without media. Driver was not able to 
recognize that
there is no media in drive.

This has been tested on both Fedora-16 x86_64 VM and Windows XP VM and the 
behaviour
is as expected, i.e. as follows:

Linux guest (Fedora 16 x86_64) tries "mount /dev/fd0" and exit with error
"mount: /dev/fd0 is not a valid block device" which is the same behavior like
bare metal with real floppy device (you have to load floppy driver at first
using e.g. "modprobe floppy" command).

For Windows XP guest the Windows floppy driver is trying to seek the virtual 
drive
when you want to open it but driver successfully detect that there is no media 
in drive
and then it's asking user to insert floppy media in the drive.

I also tested behavior of this patch if you start guest with "-nodefaults" and 
both
Windows and Linux guests detect only FDC but no drive.

Pavel

This patch has been written with help of specifications from:
http://www.ousob.com/ng/hardware/ngd127.php
http://www.isdaman.com/alsos/hardware/fdc/floppy.htm
http://wiki.osdev.org/Floppy_Disk_Controller

Signed-off-by: Pavel Hrdina<address@hidden>
Signed-off-by: Michal Novotny<address@hidden>

It would be cool to have a qtest case for this. But I think we don't
really have a nice way to talk to the qemu monitor yet, so I'm not
requesting this before the patch can go in.


---
   hw/fdc.c |   14 ++++++++++----
   hw/pc.c  |    3 ++-
   2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/hw/fdc.c b/hw/fdc.c
index a0236b7..6791eff 100644
--- a/hw/fdc.c
+++ b/hw/fdc.c
@@ -179,12 +179,14 @@ static void fd_revalidate(FDrive *drv)
       FDriveRate rate;

       FLOPPY_DPRINTF("revalidate\n");
-    if (drv->bs != NULL&&   bdrv_is_inserted(drv->bs)) {
+    if (drv->bs != NULL) {
           ro = bdrv_is_read_only(drv->bs);
           bdrv_get_floppy_geometry_hint(drv->bs,&nb_heads,&max_track,
                                         &last_sect, drv->drive,&drive,&rate);

I'm not sure how your patch works, but I believe the behaviour of
bdrv_get_floppy_geometry_hint might be one of the keys. If I understand
correctly, it will just return the default geometry, which is one for
3.5" 1.44 MB floppies, or more precisely:

{ FDRIVE_DRV_144, 20, 80, 1, FDRIVE_RATE_500K, },

Why it makes sense to have a medium geometry when there is no medium I
haven't understood yet, but last_sect/max_track = 0 didn't seem to be
enough for you. Do you know what exactly it is that makes your case work?

ro has undefined value for a BlockDriverState with no medium, but I
guess it doesn't hurt.

This modification is needed for floppy driver in guest to detect floppy drive.

My question was more about how the floppy drivers in the guest detect
drives. They can't be relying on the geometry of a medium because no
medium is inserted. So setting the geometry must have some side effect
that I'm not aware of.
Well, this is good question, I'll investigate little bit more about this and probably send new version of this patch. 
   /********************************************************/
@@ -937,6 +940,9 @@ static int fdctrl_media_changed(FDrive *drv)

       if (!drv->bs)
           return 0;
+    /* This is needed for driver to detect there is no media in drive */
+    if (!bdrv_is_inserted(drv->bs))
+        return 1;

In which case is this required to detect that there is no media? After
eject? If so, why isn't the code in fdctrl_change_cb() enough?

Or do you in fact need it for the initial state?

As i wrote to Stefan,
You can look here, http://www.isdaman.com/alsos/hardware/fdc/floppy.htm
, for specification of DIR register. Bit7 is there as CHAN and in this
bit is saved information whether media is changed or not. This bit is
set to true while there is no media. And floppy driver is checking this
bit to detect media change or media missing.

And this is needed for all cases if there is no media in drive. Code in
fdctrl_change_cb() is needed only for detect that there is no media when
you try to mount it (linux guest) or open it (windows guest).

Hm. There seems to be more wrong that just this. I think the main
problem is that we clear the bit after reading it out once, whereas it
seems to stay set in reality. It would only be cleared once some command
(not sure which are possible) succeeds with a newly inserted disk.

Unfortunately I couldn't really find any appropriate documentation for
the bit. The FDC spec says "DSKCHG monitors the pin of the same name and
reflects the opposite value seen on the disk cable", but I couldn't find
any description on how floppy drives set it on the cable.

Does anyone have some pointer to a spec for this?
This is proper behaviour, because driver will read DIR register to detect if there was media change. If it was, driver will run whatever he was doing with saved state, that there was media changed and check DIR again. If there is still bit7 setted to true, driver will decide that there is really no media in drive and end with this "error". 

Kevin
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