Subject: Re: Device driver softc questions
To: Recipient list suppressed <>
From: Steven Grunza <>
List: tech-kern
Date: 03/16/2001 09:27:05
Thank you to all who replied.  The information about auto-configuration and 
the code example:

    struct com_softc *sc = device_lookup(&com_cd, COMUNIT(dev))

are exactly what I needed.  Thanks again.

Steven G.

At 03:25 PM 3/15/2001 -0800, David Querbach wrote:
> > In the attach() routine, the second parameter is a pointer to a
> > structure that appears to be called a softc.  This structure appears to
> > hold information for the driver.  Where (in the source code) is the
> > memory for this structure allocated?
>A "softc" is the structure holding data required by a driver.  Each driver
>has its own softc structure, which is based on struct device:
>struct puc_softc {                              // dev/pci/puc.c, line 68
>         struct device           sc_dev;         // "base class" object
>         ... device-dependent data
>An instance of a softc structure is created during auto-configuration for
>each device whose match function (in your case, puc_match()) returns
>non-zero.  (Actually, that's just an approximation.  If more than one match
>function returns non-zero, the highest return value "wins".)
> > For instance in /sys/dev/pci/puc.c line 63 the definition for "struct
> > puc_softc" is created.  In line 147 of puc_attach() a local pointer
> > called sc is created on the stack and set to point to the second
> > parameter passed into the puc_attach() function.  This local stack
> > pointer is then used to update this puc_softc structure.
>Once a device is matched, the auto-configuration system malloc's space for
>the device softc as specified in the cfattach structure for the device,
>then passes it to the attach routine.  In your case:
>struct cfattach puc_ca = {                      // dev/pci/puc.c, line 99
>         sizeof(struct puc_softc), puc_match, puc_attach
>Note that this structure also lists the match and attach routines.
> > But where is a pointer to this struct puc_softc saved so the driver can
> > get to it later?  This driver doesn't have puc_open() and
> > puc_close() calls but how would they figure out where the struct
> > puc_softc is located?
>The softc pointers are stored in an array pointed to by the struct cfdriver
>for the device in question.  These structures are described in sys/device.h,
>and are actually found in ioconf.c in your kernel compilation directory.
>The file ioconf.c is created by config(8) at kernel build time.
> > For now I'm going to just use a global pointer to it but there's got to
> > be a better (more correct) way....
>Have a look at dev/ic/com.c for an example of how to find a softc:
>extern struct cfdriver com_cd;                                  // line 177
>   struct com_softc *sc = device_lookup(&com_cd, COMUNIT(dev));  // line 965
>You might also want to read the auto-configuration papers.  Look on the
> documentation page.
>David Querbach
>Real-Time Systems Inc.