Re: RFC: disk geometry via sysfs

["Seewer Philippe" <philippe.seewer@bfh.ch>]

Phillip Susi wrote:
> Seewer Philippe wrote:
> 
>> Hello all!
>>
>> I don't want to start another geometry war, but with the introduction of
>> the general getgeo function by Christoph Hellwig for all disks this
>> simply would become a matter of extending the basic gendisk block driver.
>>
>> There are people out there (like me) who need to know about disk
>> geometry. But since this is clearly post 2.6.16 I prefer to ask here
>> before writing a patch...
>>   
> 
> 
> Why do you need to know about geometry?  Geometry is a useless fiction
> that only still exists in PC system BIOS for the sake of backward
> compatibility with software that was originally designed to operate with
> MFM and RLL disks that actually used geometric addressing.  These days
> there is no such thing; it's just made up by the bios.

...Thats why I said i didn't want to start another geometry war. But
then again, I did write RFC too, yes?

Yes, geometry is a fiction. And a bad one at that. To be honest I'd
rather get rid of it completely. But you said it: The geometry still
exists for the sake of backward compatibility. If it is still there, why
not export it? That's what sysfs is for...

Additionally have a look at libata-scsi.c which is part of the SATA
implementation. Theres CHS code in there...

Personally I want the geometry information in sysfs because debugging
partition tables not written by linux tools becomes just that tad more
easier...

> 
>> Q1: Yes or No?
>> If no, the other questions do not apply
>>
>> Q2: Where under sysfs?
>> Either do /sys/block/hdx/heads, /sys/block/hdx/sectors, etc. or should
>> there be a new sub-object like /sys/block/hdx/geometry/heads?
>>   
> 
> 
> This is not suitable because block devices may not be bios accessible,
> and thus, nowhere to get any bogus geometry information from.  Even if
> it is, do we really want to be calling the bios to get this information
> and keep it around?
I did not say I'd implement it for _all_ devices. In fact I indent to
make geometry available only for devices whose drivers provide the
getgeo function.

> 
>> Q3: Writable?
>> Under some (weird) circumstances it would actually be quite nice to
>> overwrite the kernels idea of a disks geometry. This would require a
>> general function like setgeo. Acceptable?
>>
>>   
> 
> 
> What for?  The only purpose to geometry is bios compatibility.  Changing
> the kernel's copy of the values won't do any good because the bios won't
> be changed.


Exactly. I don't want the kernel to fix BIOS problems. But i want to
give userland the opportunity to overwrite what the kernel thinks (as in
/proc/ide/hdx/settings).
One example where this might be usable is connecting a PATA drive using
an Adapter to SATA. PATA returns the drive's geometry. SATA defaults to
x/255/63...
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