[PATCH v2 3/4] usb: dbgp: Fix return values for reset prep and startup

[Connor Davis <connojdavis@xxxxxxxxx>]

Callers of dbgp_reset_prep treat a 0 return value as "stop using
the debug port", which means they don't make any subsequent calls to
dbgp_reset_prep or dbgp_external_startup.

To ensure the callers' interpretation is correct, first return -EPERM
from xen_dbgp_op if !xen_initial_domain(). This ensures that
both xen_dbgp_reset_prep and xen_dbgp_external_startup return 0
iff the PHYSDEVOP_DBGP_RESET_{PREPARE,DONE} hypercalls succeed. Also
update xen_dbgp_reset_prep and xen_dbgp_external_startup to return
-EPERM when !CONFIG_XEN_DOM0 for consistency.

Next, return nonzero from dbgp_reset_prep if xen_dbgp_reset_prep returns
0. The nonzero value ensures that callers of dbgp_reset_prep will
subsequently call dbgp_external_startup when it is safe to do so.

Also invert the return values from dbgp_external_startup for
consistency with dbgp_reset_prep (this inversion has no functional
change since no callers actually check the value).

Signed-off-by: Connor Davis <connojdavis@xxxxxxxxx>
---
 drivers/usb/early/ehci-dbgp.c |  9 ++++++---
 drivers/xen/dbgp.c            |  2 +-
 include/linux/usb/ehci-dbgp.h | 14 +++++++++-----
 3 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/usb/early/ehci-dbgp.c b/drivers/usb/early/ehci-dbgp.c
index 45b42d8f6453..ff993d330c01 100644
--- a/drivers/usb/early/ehci-dbgp.c
+++ b/drivers/usb/early/ehci-dbgp.c
@@ -970,8 +970,8 @@ int dbgp_reset_prep(struct usb_hcd *hcd)
 	int ret = xen_dbgp_reset_prep(hcd);
 	u32 ctrl;
 
-	if (ret)
-		return ret;
+	if (!ret)
+		return 1;
 
 	dbgp_not_safe = 1;
 	if (!ehci_debug)
@@ -995,7 +995,10 @@ EXPORT_SYMBOL_GPL(dbgp_reset_prep);
 
 int dbgp_external_startup(struct usb_hcd *hcd)
 {
-	return xen_dbgp_external_startup(hcd) ?: _dbgp_external_startup();
+	if (!xen_dbgp_external_startup(hcd))
+		return 1;
+
+	return !_dbgp_external_startup();
 }
 EXPORT_SYMBOL_GPL(dbgp_external_startup);
 #endif /* USB */
diff --git a/drivers/xen/dbgp.c b/drivers/xen/dbgp.c
index fef32dd1a5dc..d54f98380e6f 100644
--- a/drivers/xen/dbgp.c
+++ b/drivers/xen/dbgp.c
@@ -15,7 +15,7 @@ static int xen_dbgp_op(struct usb_hcd *hcd, int op)
 	struct physdev_dbgp_op dbgp;
 
 	if (!xen_initial_domain())
-		return 0;
+		return -EPERM;
 
 	dbgp.op = op;
 
diff --git a/include/linux/usb/ehci-dbgp.h b/include/linux/usb/ehci-dbgp.h
index 62ab3805172d..c0e98557efc0 100644
--- a/include/linux/usb/ehci-dbgp.h
+++ b/include/linux/usb/ehci-dbgp.h
@@ -56,28 +56,32 @@ extern int xen_dbgp_external_startup(struct usb_hcd *);
 #else
 static inline int xen_dbgp_reset_prep(struct usb_hcd *hcd)
 {
-	return 1; /* Shouldn't this be 0? */
+	return -EPERM;
 }
 
 static inline int xen_dbgp_external_startup(struct usb_hcd *hcd)
 {
-	return -1;
+	return -EPERM;
 }
 #endif
 
 #ifdef CONFIG_EARLY_PRINTK_DBGP
-/* Call backs from ehci host driver to ehci debug driver */
+/*
+ * Call backs from ehci host driver to ehci debug driver.
+ * Returns 0 if the debug port should stopped being used,
+ * nonzero otherwise.
+ */
 extern int dbgp_external_startup(struct usb_hcd *);
 extern int dbgp_reset_prep(struct usb_hcd *);
 #else
 static inline int dbgp_reset_prep(struct usb_hcd *hcd)
 {
-	return xen_dbgp_reset_prep(hcd);
+	return !xen_dbgp_reset_prep(hcd);
 }
 
 static inline int dbgp_external_startup(struct usb_hcd *hcd)
 {
-	return xen_dbgp_external_startup(hcd);
+	return !xen_dbgp_external_startup(hcd);
 }
 #endif
 
-- 
2.31.1