Re: DRIVER_UNIDENTIFY

[John Baldwin <jhb@xxxxxxxxxxx>]

On Thursday 15 December 2005 01:13 pm, Warner Losh wrote:
> > How about creating a new driver_if.m entry point that is the complement
> > to DRIVER_IDENTIFY.  I would call it DRIVER_UNIDENTIFY() and would change
> > bus_generic_detach() to call it on each driver similar to how
> > bus_generic_probe() currently calls DRIVER_IDENTIFY() for each driver. 
> > This would allow drivers that create devices in DRIVER_IDENTIFY() have a
> > place to remove the device when they are unloaded.
>
> I have mixed feelings about this.
>
> First, the identify routine has identified a hunk of hardware and has
> placed it into the tree.  The driver disappearing doesn't change the
> fact that the hardware is still there.  Adding this new function would
> further blur the lines between attached drivers, the hardware and
> nodes in the device tree.  Device nodes are expected to be there
> unattached.  Once the device driver has detached from the device node,
> there's no harm in leaving the node arround as there are no dangling
> references.
>
> Second, the bus may be the one that decides what hardware is there.  I
> have a SOC chip that has a number of different children of its nexus
> that are always there, and will always be there.  I do not want the
> children drivers to know anything of their location, etc, since they
> cannot know because a different SOC will have the children at a
> different location.  Of course, the easy thing here is to never call
> the idenfify routine at all for this bus, but this would require
> changes to the code.
>
> However, we do currently have a assymetrical arrangement.  There's a
> way to add the device, but too many drivers are 'stupid' in how they
> add the device.  They neglect to check to make sure that the device
> hasn't already been added, which is what causes the grief.
>
> Maybe it would be better to have a better way to add instances such
> that if the idenify routine used this better way that it could be
> called many times (eg, make it idempotent).  Right now we recommend
> that driver writers do the following (which is idempotent):
>
> static void
> tscnmi_identify( driver_t* driver, device_t parent )
> {
>     devclass_t dc;
>
>     dc = devclass_find(DRIVERNAME);
>     if (devclass_get_device(dc, 0) == NULL) {
>         if (BUS_ADD_CHILD(parent, 0, DRIVERNAME, 0) == 0)
>             panic("failed to add " DRIVERNAME);
>     }
> }
>
> I think a better solution would be:
>
> static void
> tscnmi_identify( driver_t* driver, device_t parent )
> {
> 	device_add_child_once(parent, DRIVERNAME);
> }
>
> where device_add_child_once would look like the following (run through
> style(9)izer):
>
> int
> device_add_child_once(device_t parent, char *name)
> {
>     devclass_t dc;
>
>     dc = devclass_find(name);
>     if (devclass_get_device(dc, 0) == NULL)
>     	return BUS_ADD_CHILD(parent, 0, name, 0);
>     return 0;
> }

Actually, those methods enforce one instance in the system.  I want to enforce 
one instance per parent device (the example here is acpi_video which attaches 
to vgapci in my agp_cvs.patch and if you have multiple video cards, each 
might have its own acpi_video driver).  I'm about to fix acpi_video, but was 
curious if we could come up with a better overall solution.

-- 
John Baldwin <jhb@xxxxxxxxxxx>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org