Re: ez-drive and "unknown partition type 0xc"

[Jochen Hoenicke]

Hello,

Ben and I did some debugging of this problem and it turned out to be a
problem in linux's EZ remapping.  When reading the the first track,
not only the MBR gets remapped, but also all other sectors that are
included with the same read and unfortunately also the next sector,
i.e. the fat boot sector.  

I reported that to linux-kernel.  The bug is well known, but has no
clean solution and will probably not be fixed in 2.4. 

It's the best if grub does it's own work around as Andries Brouwer
suggests.  It should split reads to the first track, so that the MBR
is accessed in one read and the remaining sectors in another.  I sent
a small patch to Ben so that he can test if that works.  

  Jochen

PS: Here's the reply from linux-kernel.

On Mar 30, address@hidden wrote:
> Jochen Hoenicke writes:
> 
>     The EZ-Drive remapping code remaps to many sectors, if they are read
>     together with sector 0 in one bunch.  This is even documented:
> 
>     From linux-2.4.0/drivers/ide/ide.c line 1165:
>     /* Yecch - this will shift the entire interval,
>        possibly killing some innocent following sector */
> 
> Yes, I know - I added that comment.
> 
>     This problem hit a GRUB user using linux-2.4.2 but it exists for a
>     long time; the remapping code is already in 2.0.xx.  The reason that
>     nobody cares is probably because there are only a few programs that
>     access /dev/hda directly.
> 
>     This is what happened: Grub reads the first track in one bunch and
>     since a track has an odd number of sectors, linux adds the first
>     sector of the next track to this bunch.  This sector contains the boot
>     sector of the first FAT partition.  The result of the remapping is
>     that grub can't access that partition.
> 
> What one wants is to remap access to sector 0 to sector 1,
> and leave all other sectors alone. Thus, if someone asks
> for sectors 0 1 2 3 4, she should get sectors 1 1 2 3 4.
> Ugly, but can be done. But if someone wants to write
> sectors 0 1 2 3 4 then what? Only sectors 1 2 3 4 should be
> written, but what to write in sector 1? Nobody knows.
> (Probably a write to 0 1 2 3 4 should discard the write to 1.)
> 
> Doing this would be a very ugly wart on the IDE driver, and
> Mark Lord implemented a much smaller wart: shift transports by 1
> if they start at sector 0. This was enough at that time
> since only *fdisk and LILO access sector 0 and they do not
> read an entire track but just one sector or one block.
> 
> So yes, the problem is known, but I do not see a clean solution,
> unless the solution is to rip out all this EZ drive nonsense.
> (I can well imagine that this would happen in 2.5:
> the task of the IDE driver is to transport bits from and to
> the disk, not to worry about the contents.)
> And even if it were fixed somehow in a 2.4 kernel, lots of
> people will have a 2.2 or older system for quite some time
> to come. So probably grub should regard this as a quirk in
> the Linux handling of disks with EZ drive and adapt
> (that is, read sector 0, and then read sectors 1-N,
> but do not read 0-N).
> 
> Andries