Subject: Re: Device driver softc questions
To: Steven Grunza <email@example.com>
From: Nathan J. Williams <nathanw@MIT.EDU>
Date: 03/15/2001 18:42:26
Steven Grunza <firstname.lastname@example.org> writes:
> 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?
In the config_attach() routine in kern/subr_autoconf.c, line 363 as of
/* get memory for all device vars */
dev = (struct device *)malloc(ca->ca_devsize, M_DEVBUF,
cold ? M_NOWAIT : M_WAITOK);
"ca->ca_devsize" is from the struct cfattach that the driver declares
and that ends up in the cfdata table defined in ioconf.c.
It's worth noting that the autoconf system assumes that the first
piece of a softc is a "struct device", and Bad Things will happen if
you declare one otherwise and then step on what the autoconf system
thinks of as its data.
> 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?
By using the the device number to index into the cfdriver structure,
again defined in ioconf.c. An example from sys/dev/scsipi/sd.c (edited
down a bit):
extern struct cfdriver sd_cd;
sdclose(dev, flag, fmt, p)
int flag, fmt;
struct proc *p;
struct sd_softc *sd = sd_cd.cd_devs[SDUNIT(dev)];