V4L/DVB (9054): implement proper locking in the dvb ca en50221 driver

Concurrent access to a single DVB CA 50221 interface slot is generally
discouraged. The underlying drivers (budget-av, budget-ci) do not implement
proper locking and thus two transactions could (and do) interfere with on
another.

This fixes the following problems seen by others and myself:

 - sudden i/o errors when writing to the ci device which usually would
   result in an undefined state of the hw and require a software restart

 - errors about the CAM trying to send a buffer larger than the agreed size
   usually also resulting in an undefined state of the hw

Due the to design of the DVB CA 50221 driver, implementing the locks in the
underlying drivers would not be enough and still leave some race conditions,
even though they were harder to trigger.

Signed-off-by: Matthias Dahl <devel@mortal-soul.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/media/dvb/dvb-core/dvb_ca_en50221.h b/drivers/media/dvb/dvb-core/dvb_ca_en50221.h
index 8467e63ddc0..7df2e141187 100644
--- a/drivers/media/dvb/dvb-core/dvb_ca_en50221.h
+++ b/drivers/media/dvb/dvb-core/dvb_ca_en50221.h
@@ -45,8 +45,10 @@ struct dvb_ca_en50221 {
 	/* the module owning this structure */
 	struct module* owner;
 
-	/* NOTE: the read_*, write_* and poll_slot_status functions must use locks as
-	 * they may be called from several threads at once */
+	/* NOTE: the read_*, write_* and poll_slot_status functions will be
+	 * called for different slots concurrently and need to use locks where
+	 * and if appropriate. There will be no concurrent access to one slot.
+	 */
 
 	/* functions for accessing attribute memory on the CAM */
 	int (*read_attribute_mem)(struct dvb_ca_en50221* ca, int slot, int address);