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, 1 insertions, 0 deletions

diff --git a/include/linux/cdrom.h b/include/linux/cdrom.h
index c4fef00abdf3..0a89f111e00e 100644
--- a/include/linux/cdrom.h
+++ b/include/linux/cdrom.h
@@ -64,6 +64,7 @@ struct cdrom_device_info {
 	int for_data;
 	int (*exit)(struct cdrom_device_info *);
 	int mrw_mode_page;
+	__s64 last_media_change_ms;
 };
 
 struct cdrom_device_ops {