Per-open file private data for the cdevs

[Kostik Belousov]

On Sun, May 11, 2008 at 02:50:30PM +0300, Kostik Belousov wrote:
> On Sat, May 10, 2008 at 09:53:12PM -1000, Jeff Roberson wrote:
> > On Sun, 4 May 2008, Kostik Belousov wrote:
> > 
> > >Since the review for the clone-at-open patch (fdclone) posted some time ago
> > >mostly says that it would be better to implement per-file private data
> > >instead, I produced the patch along this line,
> > >
> > >The patch does not change the cdevsw ABI, instead, three new functions
> > >nt	devfs_get_cdevpriv(void **datap);
> > >int	devfs_set_cdevpriv(void *priv, cdevpriv_dtr_t dtr);
> > >void	devfs_clear_cdevpriv(void);
> > >are provided for manipulation of the per-file private data.
> > >
> > >devfs_set_cdevpriv assigns the priv as private data for the file descriptor
> > >which is used to initiate currently performed driver operation. dtr
> > >is the function that will be called when either the last refernce to
> > >the file goes away or devfs_clear_cdevpriv is called.
> > >
> > >devfs_get_cdevpriv is the obvious accessor.
> > >
> > >devfs_clear_cdevpriv allows to clear the private data for the still
> > >open file.
> > >
> > >The synchronization of the cdev data and file private data is left
> > >to the driver code, I did not found any generic helper mechanism that
> > >could be useful there.
> > >
> > >Patch:
> > >http://people.freebsd.org/~kib/misc/fdpriv.1.patch
> > >
> > >Dumb driver that shows the basic usage of the proposed KPI:
> > >http://people.freebsd.org/~kib/misc/fpclone.c
> > >
> > >Previous version of the patch was tested by Peter Holm.
> > >
> > 
> > Hi Kostik,
> > 
> > Are these per-instances structures intended to be used by anything other 
> > than devices?  If not can we make them a union with the DTYPE_VNODE 
> > fields to save space?
> > 
> > Thanks,
> > Jeff
> 
> The current version of the patch is at
> http://people.freebsd.org/~kib/misc/fdpriv.3.patch
> 
> Per insistence of John Baldwin and request of Eric Anholt, the destructors
> are called now when either file is last closed, or the device is destroyed.
> This versions adds only one pointer to the struct file.
> 
> Jeff, would you, please, explicitely specify what field you propose to
> union with the f_cdevpriv ? 

Of course, I forgot to answer the question. Yes, the KPI is supposed to
be used by the drivers only. Please, note that device open files have
DTYPE_VNODE.
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