Add a #template device

This is for quickly crafting simple devices that use a devdir table and
devgen().

Signed-off-by: Barret Rhoden <brho@cs.berkeley.edu>

kern/drivers/dev/template.c

diff --git a/kern/drivers/dev/template.c b/kern/drivers/dev/template.c
new file mode 100644
index 0000000..5d92116
--- /dev/null
+++ b/kern/drivers/dev/template.c
@@ -0,0 +1,141 @@
+/* Copyright (c) 2020 Google Inc
+ * Barret Rhoden <brho@cs.berkeley.edu>
+ * See LICENSE for details.
+ *
+ * #template.  Dummy device with a devdir table.
+ */
+
+#include <ns.h>
+#include <kmalloc.h>
+#include <string.h>
+#include <stdio.h>
+#include <assert.h>
+#include <error.h>
+
+struct dev template_devtab;
+
+static char *devname(void)
+{
+	return template_devtab.name;
+}
+
+enum {
+	Qdir,
+	Qctl,
+};
+
+static struct dirtab XXX_dir[] = {
+	{".", {Qdir, 0, QTDIR}, 0, DMDIR | 0555},
+	{"ctl", {Qctl, 0, QTFILE}, 0, 0666},
+};
+
+static struct chan *XXX_attach(char *spec)
+{
+	return devattach(devname(), spec);
+}
+
+static struct walkqid *XXX_walk(struct chan *c, struct chan *nc, char **name,
+			       unsigned int nname)
+{
+	return devwalk(c, nc, name, nname, XXX_dir, ARRAY_SIZE(XXX_dir),
+		       devgen);
+}
+
+static size_t XXX_stat(struct chan *c, uint8_t *db, size_t n)
+{
+	return devstat(c, db, n, XXX_dir, ARRAY_SIZE(XXX_dir), devgen);
+}
+
+static struct chan *XXX_open(struct chan *c, int omode)
+{
+	return devopen(c, omode, XXX_dir, ARRAY_SIZE(XXX_dir), devgen);
+}
+
+static void XXX_close(struct chan *c)
+{
+	/* If you do anything after open, only undo it for COPEN chans */
+	if (!(c->flag & COPEN))
+		return;
+}
+
+static size_t XXX_read(struct chan *c, void *ubuf, size_t n, off64_t offset)
+{
+	switch (c->qid.path) {
+	case Qdir:
+		return devdirread(c, ubuf, n, XXX_dir, ARRAY_SIZE(XXX_dir),
+				  devgen);
+	case Qctl:
+		return readstr(offset, ubuf, n, "XXX");
+	default:
+		panic("Bad Qid %p!", c->qid.path);
+	}
+	return -1;
+}
+
+#define XXX_CTL_USAGE "start|stop|print|reset"
+
+static void XXX_ctl_cmd(struct chan *c, struct cmdbuf *cb)
+{
+	ERRSTACK(1);
+
+	if (cb->nf < 1)
+		error(EFAIL, XXX_CTL_USAGE);
+
+	if (waserror()) {
+		nexterror();
+	}
+	if (!strcmp(cb->f[0], "start")) {
+		;
+	} else if (!strcmp(cb->f[0], "stop")) {
+		;
+	} else if (!strcmp(cb->f[0], "print")) {
+		;
+	} else if (!strcmp(cb->f[0], "reset")) {
+		;
+	} else {
+		error(EFAIL, XXX_CTL_USAGE);
+	}
+	poperror();
+}
+
+static size_t XXX_write(struct chan *c, void *ubuf, size_t n, off64_t unused)
+{
+	ERRSTACK(1);
+	struct cmdbuf *cb = parsecmd(ubuf, n);
+
+	if (waserror()) {
+		kfree(cb);
+		nexterror();
+	}
+	switch (c->qid.path) {
+	case Qctl:
+		XXX_ctl_cmd(c, cb);
+		break;
+	default:
+		error(EFAIL, "Unable to write to %s", devname());
+	}
+	kfree(cb);
+	poperror();
+	return n;
+}
+
+struct dev template_devtab __devtab = {
+	.name = "template",
+	.reset = devreset,
+	.init = devinit,
+	.shutdown = devshutdown,
+	.attach = XXX_attach,
+	.walk = XXX_walk,
+	.stat = XXX_stat,
+	.open = XXX_open,
+	.create = devcreate,
+	.close = XXX_close,
+	.read = XXX_read,
+	.bread = devbread,
+	.write = XXX_write,
+	.bwrite = devbwrite,
+	.remove = devremove,
+	.wstat = devwstat,
+	.power = devpower,
+	.chaninfo = devchaninfo,
+};