[PATCH v3] drivers/cdrom: improved ioctl for media change detection

[Lukas Prediger]

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@xxxxxxxx>
---
I have now incorporated the suggestions of Phil, Christoph and Randy for v2.
- fixed overly long lines
- has_changed is now media_flags in the new ioctl struct
- fixed brace on new line after struct definition
- using if instead of cryptic assignment for better readability

Hope everyone is happy with it now. However, if anything still comes up, let me know.
Thanks for all the feedback so far.

Best,
Lukas
---
 Documentation/cdrom/cdrom-standard.rst | 11 +++++
 Documentation/ioctl/cdrom.rst          |  3 ++
 drivers/cdrom/cdrom.c                  | 59 ++++++++++++++++++++++++--
 include/linux/cdrom.h                  |  1 +
 include/uapi/linux/cdrom.h             | 19 +++++++++
 5 files changed, 89 insertions(+), 4 deletions(-)

diff --git a/Documentation/cdrom/cdrom-standard.rst b/Documentation/cdrom/cdrom-standard.rst
index dde4f7f7fdbf..4d69515df864 100644
--- a/Documentation/cdrom/cdrom-standard.rst
+++ b/Documentation/cdrom/cdrom-standard.rst
@@ -923,6 +923,17 @@ 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_.
diff --git a/Documentation/ioctl/cdrom.rst b/Documentation/ioctl/cdrom.rst
index 3b4c0506de46..bac5bbf93ca0 100644
--- a/Documentation/ioctl/cdrom.rst
+++ b/Documentation/ioctl/cdrom.rst
@@ -54,6 +54,9 @@ are as follows:
 	CDROM_SELECT_SPEED	Set the CD-ROM speed
 	CDROM_SELECT_DISC	Select disc (for juke-boxes)
 	CDROM_MEDIA_CHANGED	Check is media changed
+	CDROM_TIMED_MEDIA_CHANGE	Check if media changed
+					since given time
+					(struct cdrom_timed_media_change_info)
 	CDROM_DRIVE_STATUS	Get tray position, etc.
 	CDROM_DISC_STATUS	Get disc type, etc.
 	CDROM_CHANGER_NSLOTS	Get number of slots
diff --git a/drivers/cdrom/cdrom.c b/drivers/cdrom/cdrom.c
index ac42ae4651ce..0bc811a629e1 100644
--- a/drivers/cdrom/cdrom.c
+++ b/drivers/cdrom/cdrom.c
@@ -344,6 +344,12 @@ static void cdrom_sysctl_register(void);
 
 static LIST_HEAD(cdrom_list);
 
+static void signal_media_change(struct cdrom_device_info *cdi)
+{
+	cdi->mc_flags = 0x3; /* set media changed bits, on both queues */
+	cdi->last_media_change_ms = ktime_to_ms(ktime_get());
+}
+
 int cdrom_dummy_generic_packet(struct cdrom_device_info *cdi,
 			       struct packet_command *cgc)
 {
@@ -613,6 +619,7 @@ int register_cdrom(struct cdrom_device_info *cdi)
 	ENSURE(cdo, generic_packet, CDC_GENERIC_PACKET);
 	cdi->mc_flags = 0;
 	cdi->options = CDO_USE_FFLAGS;
+	cdi->last_media_change_ms = ktime_to_ms(ktime_get());
 
 	if (autoclose == 1 && CDROM_CAN(CDC_CLOSE_TRAY))
 		cdi->options |= (int) CDO_AUTO_CLOSE;
@@ -1413,8 +1420,7 @@ static int cdrom_select_disc(struct cdrom_device_info *cdi, int slot)
 		cdi->ops->media_changed(cdi, slot);
 
 	if (slot == CDSL_NONE) {
-		/* set media changed bits, on both queues */
-		cdi->mc_flags = 0x3;
+		signal_media_change(cdi);
 		return cdrom_load_unload(cdi, -1);
 	}
 
@@ -1447,7 +1453,7 @@ static int cdrom_select_disc(struct cdrom_device_info *cdi, int slot)
 		slot = curslot;
 
 	/* set media changed bits on both queues */
-	cdi->mc_flags = 0x3;
+	signal_media_change(cdi);
 	if ((ret = cdrom_load_unload(cdi, slot)))
 		return ret;
 
@@ -1516,7 +1522,7 @@ int media_changed(struct cdrom_device_info *cdi, int queue)
 		changed = cdi->ops->media_changed(cdi, CDSL_CURRENT);
 
 	if (changed) {
-		cdi->mc_flags = 0x3;    /* set bit on both queues */
+		signal_media_change(cdi);
 		ret |= 1;
 		cdi->media_written = 0;
 	}
@@ -2389,6 +2395,49 @@ static int cdrom_ioctl_media_changed(struct cdrom_device_info *cdi,
 	return ret;
 }
 
+/*
+ * Media change detection with timing information.
+ *
+ * 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 (in ms) of the last known media change (by the caller).
+ * Upon successful return, ioctl call will set arg->last_media_change
+ * to the latest media change timestamp 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.
+ */
+static int cdrom_ioctl_timed_media_change(struct cdrom_device_info *cdi,
+		unsigned long arg)
+{
+	int ret;
+	struct cdrom_timed_media_change_info __user *info;
+	struct cdrom_timed_media_change_info tmp_info;
+
+	if (!CDROM_CAN(CDC_MEDIA_CHANGED))
+		return -ENOSYS;
+
+	info = (struct cdrom_timed_media_change_info __user *)arg;
+	cd_dbg(CD_DO_IOCTL, "entering CDROM_TIMED_MEDIA_CHANGE\n");
+
+	ret = cdrom_ioctl_media_changed(cdi, CDSL_CURRENT);
+	if (ret < 0)
+		return ret;
+
+	if (copy_from_user(&tmp_info, info, sizeof(tmp_info)) != 0)
+		return -EFAULT;
+
+	tmp_info.media_flags = 0;
+	if (tmp_info.last_media_change - cdi->last_media_change_ms < 0)
+		tmp_info.media_flags |= MEDIA_CHANGED_FLAG;
+
+	tmp_info.last_media_change = cdi->last_media_change_ms;
+
+	if (copy_to_user(info, &tmp_info, sizeof(*info)) != 0)
+		return -EFAULT;
+
+	return 0;
+}
+
 static int cdrom_ioctl_set_options(struct cdrom_device_info *cdi,
 		unsigned long arg)
 {
@@ -3340,6 +3389,8 @@ int cdrom_ioctl(struct cdrom_device_info *cdi, struct block_device *bdev,
 		return cdrom_ioctl_eject_sw(cdi, arg);
 	case CDROM_MEDIA_CHANGED:
 		return cdrom_ioctl_media_changed(cdi, arg);
+	case CDROM_TIMED_MEDIA_CHANGE:
+		return cdrom_ioctl_timed_media_change(cdi, arg);
 	case CDROM_SET_OPTIONS:
 		return cdrom_ioctl_set_options(cdi, arg);
 	case CDROM_CLEAR_OPTIONS:
diff --git a/include/linux/cdrom.h b/include/linux/cdrom.h
index 528271c60018..165d2c803670 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 {
diff --git a/include/uapi/linux/cdrom.h b/include/uapi/linux/cdrom.h
index 2817230148fd..0c7043b5607f 100644
--- a/include/uapi/linux/cdrom.h
+++ b/include/uapi/linux/cdrom.h
@@ -147,6 +147,8 @@
 #define CDROM_NEXT_WRITABLE	0x5394	/* get next writable block */
 #define CDROM_LAST_WRITTEN	0x5395	/* get last block written on disc */
 
+#define CDROM_TIMED_MEDIA_CHANGE   0x5396  /* get the timestamp of the last media change */
+
 /*******************************************************
  * CDROM IOCTL structures
  *******************************************************/
@@ -292,6 +294,23 @@ struct cdrom_generic_command
 	void			__user *reserved[1];	/* unused, actually */
 };
 
+/* This struct is used by CDROM_TIMED_MEDIA_CHANGE */
+struct cdrom_timed_media_change_info {
+	__s64	last_media_change;	/* Timestamp of the last detected media
+					 * change in ms. May be set by caller,
+					 * updated upon successful return of
+					 * ioctl.
+					 */
+	__u64	media_flags;		/* Flags returned by ioctl to indicate
+					 * media status.
+					 */
+};
+#define MEDIA_CHANGED_FLAG	0x1	/* Last detected media change was more \
+					 * recent than last_media_change set by\
+					 * caller.                             \
+					 */
+/* other bits of media_flags available for future use */
+
 /*
  * A CD-ROM physical sector size is 2048, 2052, 2056, 2324, 2332, 2336, 
  * 2340, or 2352 bytes long.  
-- 
2.25.1