media: dvb_frontend: better document the -EPERM condition

Two readonly ioctls can't be allowed if the frontend device
is opened in read only mode. Explain why.

Reviewed by: Shuah Khan <shuahkh@osg.samsung.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

diff --git a/drivers/media/dvb-core/dvb_frontend.c b/drivers/media/dvb-core/dvb_frontend.c
index 45d0d488dfcbf2c9930f958f67bdcea13306b4ce..b19f40be0ab2374cbc6422454b25a1b666a4d1ac 100644 (file)
--- a/drivers/media/dvb-core/dvb_frontend.c
+++ b/drivers/media/dvb-core/dvb_frontend.c
@@ -1923,9 +1923,23 @@ static int dvb_frontend_ioctl(struct file *file, unsigned int cmd, void *parg)
                return -ENODEV;
        }
 
-       if ((file->f_flags & O_ACCMODE) == O_RDONLY &&
-           (_IOC_DIR(cmd) != _IOC_READ || cmd == FE_GET_EVENT ||
-            cmd == FE_DISEQC_RECV_SLAVE_REPLY)) {
+       /*
+        * If the frontend is opened in read-only mode, only the ioctls
+        * that don't interfere with the tune logic should be accepted.
+        * That allows an external application to monitor the DVB QoS and
+        * statistics parameters.
+        *
+        * That matches all _IOR() ioctls, except for two special cases:
+        *   - FE_GET_EVENT is part of the tuning logic on a DVB application;
+        *   - FE_DISEQC_RECV_SLAVE_REPLY is part of DiSEqC 2.0
+        *     setup
+        * So, those two ioctls should also return -EPERM, as otherwise
+        * reading from them would interfere with a DVB tune application
+        */
+       if ((file->f_flags & O_ACCMODE) == O_RDONLY
+           && (_IOC_DIR(cmd) != _IOC_READ
+               || cmd == FE_GET_EVENT
+               || cmd == FE_DISEQC_RECV_SLAVE_REPLY)) {
                up(&fepriv->sem);
                return -EPERM;
        }