Linux-kernel mailing list archive 1999-23,: Re: Devfs, was Re: Migrating to larger numbers

# Re: Devfs, was Re: Migrating to larger numbers

Richard Gooch (rgooch@atnf.csiro.au)
Wed, 9 Jun 1999 17:47:01 +1000

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Guest section writes:
> From: Richard Gooch <rgooch@atnf.csiro.au>
>
> Guest section writes:
> > From: Richard Gooch <rgooch@atnf.csiro.au>
> >
> > However, with devfs you can definately avoid the lookups into the
> > major tables. Right now that's fast because they're simple arrays. If
> > we go to 12 bit majors (or more), we're going to want to set them up
> > as lists of some kind. And that means some kind of searching/hashing.
> > And that's all for the benefit of finding our fops. Devfs completely
> > avoids this overhead.
> >
> > No. A kdev_t is a pointer to a structure that has various fields.
> > Not:
> > blksize[MAJOR(dev)]
> > but
> > dev->major->blksize
>
> Cute. All, right, if you want to define kdev_t that way, go ahead. It
> now has no relation to i_rdev. So once again we go through a lookup.
>
> I am not sure what you mean by `no relation to i_rdev'.
> Indeed, i_rdev is of type kdev_t.
>
> Of course there is a lookup somewhere, namely at the moment
> the device node is opened.

Exactly my point. The device number stored in the inode is just a
number. Then you have to lookup the corresponding kdev_t
structure. In fact what happens is we lookup by major. Right now with
8 bit majors we have a table. If we go to 12 or 16 bit majors, we'll
have a list instead. So the lookup is no longer trivial.
Devfs completely avoids this.

Regards,

Richard....

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