On Wed, 17 Feb 1999, Colin Wood wrote:

> davem@eastcoast.co.za wrote:
> > Summary:
> > I'm trying to write a trivial pseudo device driver for learning 
> > purposes. I'll progress to something more serious at a later stage.
> > My driver is called mypseudo.
> > 
> > Do I need a mypseudo_softc or don't I? Some drivers seem to have 
> > them and others dont. Is it lake taking sugar in your 
> > coffee?:-) or is there a deeper reason that some do and some dont?
> 
> Basically, if you have more than 1 instance of the device, you're going to
> need a softc structure.  Even if you're not going to have one, it might
> still be a good idea.
>  
> > Also, my (limited) understanding is that it should look like:
> > 
> > struct pseudo_softc {
> >     struct device sc_dev;
> >     /*Other stuff here*/
> > }
> 
> This is correct.
> 
> > and yet (for e.g.), I see in arch/x68k/dev/kbd.c
> > 
> > struct kbd_softc {
> > 	int k_event_mode; /* if true, collect events, else pass to ite */
> > 	struct evvar k_events; /* event queue state */
> > } kbd_softc;
> > 
> > i.e. the first field is not a "struct device".
> 
> I'd assume that in this case, they only have 1 instance of the device, and
> they're not actually using the softc the way you should use it.  Also,
> they probably have a copy of a struct device floating around somewhere
> else, since you have to have a struct device in order to go through the
> autoconfiguration process.

Actually pseudo-devices short-circuit autoconfig. When you add a
pseudo-device foo # line to a kernel config, an entry in the pdevinit
structure (in ioconf.c) gets generated listing fooattach and the #. At one
point in the startup, each routine listed gets called with # as a
parameter. So pseudodevices don't need a struct dev. They are a bit
different that way. ;-)

> > At this stage, I'm quite happy for there to be only one instance of 
> > the driver (no. of drivers seem to have something to do with it). If I 
> > could just get it compiled into the Kernel and be able to enter it 
> > appropriately, I'd be a happy man! (well for a day or so anyway:-)
> 
> basically, a softc structure is a data structure that is stored in a known
> location in the device tree and contains the state your driver needs to
> function.  i guess you could just use global variables (or even static
> ones inside a single file), but if you ever needed more than 1 instance of
> the device, then 2 or more of them could stomp on each other's internal
> state.  rather nasty :-)
> 
> so, figure out what kind of state your device is going to need, put it in
> a softc structure, and put that structure and any constants it needs in a
> header file (usually <device>var.h).  if you're attaching to a bus
> (instead of being a psuedo-device), you might also need some kind of
> attach_args structure to carry match/probe data around, but you can worry
> about that later.
> 
> anyway, you also need to remember to create a cfattach structure which has
> enough room for your softc and pointers to your match/probe and attach
> functions.  of course, a psuedo-device might not need a cfattach struture.

Right. pseudodevs don't need cfattach structures.

Take care,

Bill