devfs_get/set_cdevpriv functions from FreeBSD

[Markus Pfeiffer]

Dear all,
(I think in particular Matt and Alex H.)

while porting drivers from FreeBSD some of us have stumbled over the functions
devfs_get_cdevpriv and devfs_set_cdevpriv, which store some
per-open-filedescriptor information for some devices.

I looked into the FreeBSD code and the implementation seems weird to me since
they store the filedescriptor on syscall in their thread struct and the functions
rely on that hidden state.

Question is now: Should we have similar functionality (i.e. per open
filedescriptor "private" data) and if yes how to best implement that. I know
that AlexH had made some suggestion but I cannot find it in the IRC logs and
it would probably be more efficient doing this by mail.

Markus
--
Markus Pfeiffer, University of St Andrews
email: markus.pfeiffer@... | xmpp: markus.pfeiffer@...

[Francois Tigeot-2]

Hi,

On Tue, Jul 17, 2012 at 08:11:13AM +0000, Markus Pfeiffer wrote:

>
> while porting drivers from FreeBSD some of us have stumbled over the functions
> devfs_get_cdevpriv and devfs_set_cdevpriv, which store some
> per-open-filedescriptor information for some devices.
>
> I looked into the FreeBSD code and the implementation seems weird to me since
> they store the filedescriptor on syscall in their thread struct and the functions
> rely on that hidden state.
>
> Question is now: Should we have similar functionality (i.e. per open
> filedescriptor "private" data) and if yes how to best implement that. I know
> that AlexH had made some suggestion but I cannot find it in the IRC logs and
> it would probably be more efficient doing this by mail.

I encountered the same family of functions while looking at recent FreeBSD DRM
code; since it was ported from Linux in the first place, I also looked at the
Linux equivalent functions, and they seemed much more sane.

Private data was directly accessible from the filedescriptor if I remember
correctly.

--
Francois Tigeot

[Alex Hornung]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 17/07/12 09:11, Markus Pfeiffer wrote:
> Question is now: Should we have similar functionality (i.e. per
> open filedescriptor "private" data) and if yes how to best
> implement that. I know that AlexH had made some suggestion but I
> cannot find it in the IRC logs and it would probably be more
> efficient doing this by mail.

My original suggestion, which is straight forward to implement, is to
just store the fp in each of the devfs_fo_* operations somewhere (e.g.
curthread like FreeBSD, although that isn't all that nice), and then
clean it up on devfs_fo_close.

Having that, you can easily implement the rest to set up some list or
tree or whatever inside a  cdev that is also passed to the function.
the search key would then be the fp that you stored somewhere whilst
in devfs_fo_*.

HTH,
Alex
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.18 (GNU/Linux)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/

iQEcBAEBAgAGBQJQBpD+AAoJEInr8DonK72KMuEIAIa2/rGQeMivoWj19iFQkbWH
3Qi/bw6HeQ8392B5lG7P5c1eY9zxJCZZnME4vrUGX+IaWepXlZyHfsosFQ6KyiTu
K4GIm5HSLBATS9vAgQotZ7yHyExy+m54AZ6C4mXkwK4lU8/5+Gi4wV6D/8lg4Fco
pxKjDL9IOXUUQmXvQ6bC5xu8uX8JKuobO3N6LSjJpHwQkHb89fTi2ELh18aOxV6G
uh1BBu4AQFtlDC9wGb/5599wAAGTzSDdvzSBkADs4ckhh204xBlNMEQgBT+f1QCf
BuEtIgifU10nqMVd9fPbNMzEfC0mCq8JVOsgBrOGqjyZG+1upLNM8QhuokoYO8g=
=C91L
-----END PGP SIGNATURE-----

[Francois Tigeot-2]

On Tue, Jul 17, 2012 at 08:11:13AM +0000, Markus Pfeiffer wrote:

>
> while porting drivers from FreeBSD some of us have stumbled over the functions
> devfs_get_cdevpriv and devfs_set_cdevpriv, which store some
> per-open-filedescriptor information for some devices.
>
> I looked into the FreeBSD code and the implementation seems weird to me since
> they store the filedescriptor on syscall in their thread struct and the functions
> rely on that hidden state.
>
> Question is now: Should we have similar functionality (i.e. per open
> filedescriptor "private" data) and if yes how to best implement that.

Have a look at our existing drm infrastructure; there is already some code
doing this (a direct replacement of the FreeBSD version).

--
Francois Tigeot