drivers/cdrom: improved ioctl for media change detection

The current implementation of the CDROM_MEDIA_CHANGED ioctl relies on
global state, meaning that only one process can detect a disc change
while the ioctl call will return 0 for other calling processes afterwards
(see bug 213267).

This introduces a new cdrom ioctl, CDROM_TIMED_MEDIA_CHANGE, that
works by maintaining a timestamp of the last detected disc change instead
of a boolean flag: Processes calling this ioctl command can provide
a timestamp of the last disc change known to them and receive
an indication whether the disc was changed since then and the updated
timestamp.

I considered fixing the buggy behavior in the original
CDROM_MEDIA_CHANGED ioctl but that would require maintaining state
for each calling process in the kernel, which seems like a worse
solution than introducing this new ioctl.

Signed-off-by: Lukas Prediger <lumip@lumip.de>
Link: https://lore.kernel.org/all/20210912191207.74449-1-lumip@lumip.de
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210913230942.1188-1-phil@philpotter.co.uk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 11 insertions, 0 deletions

diff --git a/Documentation/cdrom/cdrom-standard.rst b/Documentation/cdrom/cdrom-standard.rst
index 5845960ca382..52ea7b6b2fe8 100644
--- a/Documentation/cdrom/cdrom-standard.rst
+++ b/Documentation/cdrom/cdrom-standard.rst
@@ -907,6 +907,17 @@ commands can be identified by the underscores in their names.
 	specifies the slot for which the information is given. The special
 	value *CDSL_CURRENT* requests that information about the currently
 	selected slot be returned.
+`CDROM_TIMED_MEDIA_CHANGE`
+	Checks whether the disc has been changed since a user supplied time
+	and returns the time of the last disc change.
+
+	*arg* is a pointer to a *cdrom_timed_media_change_info* struct.
+	*arg->last_media_change* may be set by calling code to signal
+	the timestamp of the last known media change (by the caller).
+	Upon successful return, this ioctl call will set
+	*arg->last_media_change* to the latest media change timestamp (in ms)
+	known by the kernel/driver and set *arg->has_changed* to 1 if
+	that timestamp is more recent than the timestamp set by the caller.
 `CDROM_DRIVE_STATUS`
 	Returns the status of the drive by a call to
 	*drive_status()*. Return values are defined in cdrom_drive_status_.