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media: dvb_frontend: better document the -EPERM condition

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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>
This commit is contained in:
Mauro Carvalho Chehab 2017-09-18 08:05:42 -04:00
parent ceb22c8eee
commit da5516b5e8
1 changed files with 17 additions and 3 deletions
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20
drivers/media/dvb-core/dvb_frontend.c
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@ -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;
}