Linux-kernel mailing list archive 1999-41,: RE: [linux-usb] Re: USB device allocation

# RE: [linux-usb] Re: USB device allocation

Richard B. Johnson (root@chaos.analogic.com)
Tue, 12 Oct 1999 08:28:49 -0400 (EDT)

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On Fri, 8 Oct 1999, David Waite wrote:

> > -----Original Message-----
> > From: Richard B. Johnson [mailto:root@chaos.analogic.com]
>
> <some snippage>
> > To give a simple example, given:
> >
> > LargeFileNameDirectory/LFN-Subdirectory/MyFile.txt
> >
> > ... requires 3 directory lookups to obtain a number. After which,
> > the file is accessed as a number. It is read/written/extended/truncated,
> > /seeked, etc, without ever having to parse strings again.
> >
> > The same is true for 'special files'. These require a lookup first
> > to associate a major/minor number (instead of an inode). Then the
> > human readable name string is never accessed again. To do it any other
> > way would be to return to the days before even CP/M.
> >
> > Historical buffs may remember that the file-system on the famous
> > "green-machine" (MDS-200), circa 1967, consisted of 'directory' entries
> > that contained only numbers. A directory program translated these numbers
> > into strings in a "container file". (No M$ didn't invent the container
> > file). Even then, it was understood that strings didn't belong within
> > an operating system.
> >
> >
> > So, we are not "fumbling around" with the major/minor concept. It
> > is an excellent, efficient way of handling devices. However, soon
> > it will have to be extended because we need more numbers.
>
> I think I am beginning to understand (sorry, I am definately _not_ an OS
> expert :)). But after a file is open'ed, does it even matter how the user
> got there? I am under the impression that an open'ed character device is
> just a pipe to the respective hardware driver. After you have a file
> descriptor, it doesn't matter if there was a major/minor number associated
> at all.
>
> But what a major/minor does buy is an easy transition from a file to the
> respective subsystem. 192/12 might mean 'the usb driver, 12th device'. After
> everything is in place, it really doesn't matter that we used the
> major/minor pair at all to open things up.
>
[SNIPPED]

What does matter is that the drivers use the major number to
represent what they are, and the minor number as either a particular
instance of a device or a particular parameter, depending upon the
device. Look at /dev/fdxxxx (major/minor) as an example.

If these devices used strings, asside from having to all be rewritten,
you need a variable-length string-lookup every time they are accessed.

Cheers,
Dick Johnson
**** FILE SYSTEM WAS MODIFIED ****
Penguin : Linux version 2.3.13 on an i686 machine (400.59 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.

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