linux-kernel - Re: What to do about the 2TB limit on HDIO

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Message-ID: <47E99A02.7040903@rtr.ca>
Date:	Tue, 25 Mar 2008 20:34:10 -0400
From:	Mark Lord <lkml@....ca>
To:	Greg KH <gregkh@...e.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...nel.dk>,
	Jeff Garzik <jgarzik@...ox.com>, Tejun Heo <htejun@...il.com>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: What to do about the 2TB limit on HDIO_GETGEO ?

Greg KH wrote:
> On Tue, Mar 25, 2008 at 01:37:03PM -0400, Mark Lord wrote:
>> Perhaps Greg will chime in.
> 
> I've been waiting to see if sanity will take hold of anyone here.
..

So have we.  sysfs is a total nightmare to extract information from
under program / script control.  The idea presented in this thread,
is to have it cross-index the contents with a method that actually
makes it easy to access in many common scenarios, without requiring
huge gobs of code in user space.  Or in kernel space.

And it's not just a few 10s of lines of code currently,
but rather about 80-100 lines just to find the correct device subdir,
and *then* a few more 10s of lines of code to retrieve the value.

In a bulletproof fashion, that is.  Sure it can be slightly smaller
if niceties such as error checking/handling are omitted.

There's no guarantee that udev is present, and even if it were present,
there's no guarantee that the names in /dev/ will match /sysfs/ pathnames,
since udev is very configurable to do otherwise.

So lookups are by dev_t, which sysfs has no simple or even easy way
of accomplishing.  O(n) at a minimum.

If we make it easier to access, then more programs will use it
rather than us having to expand our tricky binary ioctl interfaces.

Isn't that part of the idea of sysfs -- to limit the need for new ioctls ?

Cheers
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