Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

[Phillip Potter]

On Wed, Sep 08, 2021 at 05:42:30PM -0700, Randy Dunlap wrote:
> Documentation/process/coding-style.rst says:
> 
>   The preferred limit on the length of a single line is 80 columns.
> 
> checkpatch only checks lines > 100 columns since that is OK in a few
> cases, like a long quoted string.
> 
> So try to limit line lengths to 80 columns unless there is some
> other reason not to do that.
> 

Dear Randy,

Thank you for clarifying this, appreciate it. Will try and bear it in
mind in future :-)

Regards,
Phil

Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

[Phillip Potter]

On Thu, Sep 09, 2021 at 09:05:54PM +0300, Lukas Prediger wrote:
> Dear Christoph, Phillip and Randy,
> 
> thanks to you all for your comments!
> 

Dear Lukas,

You're welcome, thank you for the code.

> >>> Overly long line here, but more importantly this is much cleaner with
> >>> a good old if:
> >>>
> >>>
> >>> 	if (tmp_info.last_media_change - cdi->last_media_change_ms) < 0)
> >>> 		tmp_info.has_changed = 1;
> >>>
> >> 
> >> Whilst I don't disagree this is technically cleaner, the existing style
> >> certainly read well to me. 
> 
> The if would additionally require to explicitly initialise .has_changed to
> zero for the else case, so I favored the single assignment that covers
> all cases. I don't have a strong opinion on this, though, so if the if variant
> is generally favored, I can change this. (And I will definitely fix the overlength).
>

Yes true, but I guess your existing style is harder to split across
lines in a clean way. As mentioned, I didn't mind the original code, but
the line length is a fair point. Your call on this one - those with far
more experience than me would probably argue the if/else form though.

> >> In terms of line length, checkpatch doesn't
> >> complain about it, so I guess you mean purely from a visual perspective?
> >
> > Documentation/process/coding-style.rst says:
> >
> >    The preferred limit on the length of a single line is 80 columns.
> >
> > checkpatch only checks lines > 100 columns since that is OK in a few
> > cases, like a long quoted string.
> >
> > So try to limit line lengths to 80 columns unless there is some
> > other reason not to do that.
> 
> I wasn't aware that checkpatch.pl does not complain if I exceed the 80 cols,
> have fixed those now for an upcoming resubmission.
> 

Same, guilty as charged on this one - live and learn I guess :-)

> >>> +{
> >>> +	__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	has_changed;		/* Set to 1 by ioctl if last detected media
> >>>
> >>> More overly long lines.  Also why is has_changed a u64 if it is used as
> >>> a boolean flag?
> >>
> >> As this is not a packed struct, would not a smaller value still take up
> >> the same space?
> >
> > Might as well be explicit about it and also make it obvious that there
> > is some space available for other fields.
> 
> I had this as a __u8 in the first submission but Jens asked me to change it.
> From his feedback on this:
> 
> "The struct layout should be modified such that there are no holes or
> padding in it. Probably just make the has_changed a flags thing, and
> make it u64 as well. Then you can define bit 0 to be HAS_CHANGED, and
> that leaves you room to add more flags in the future."
> https://lore.kernel.org/lkml/6d6c533d-465e-33ee-5801-cb7ea84924a8@kernel.dk/
> 

Yeah, maybe just a bit more in the comment to emphasize the room for
extra bits in has_changed? I agree it looks fine like this to me though
given the lack of struck packing anyway.

> I changed it to __u64 to address this. We could think about turning it
> back to a __u8 (or bool) and add some explicit padding members
> (a __u8 reserved[3]?), but honestly I don't see much real benefit in that
> compared to how it is now.
> 

I agree with you on this personally, I think it's fine.

Regards,
Phil

Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

[Phillip Potter]

On Thu, Sep 09, 2021 at 06:25:35PM -0700, Randy Dunlap wrote:
> I have no problem with Jens's suggestion.  It would look more like this:
> 
> +{
> +	__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;		/* various <struct> flags */
> 
> #define MEDIA_CHANGED			0x1 /* Set to 1 by ioctl if last detected media */
> /* other bits of media_flags available for future use */
> 
> 
> and not having __u64 has_changed;
> which is overkill for a flag.
> 
> -- 
> ~Randy
> 

Yeah I like this, good idea. Thanks.

Regards,
Phil

Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

[Phillip Potter]

On Thu, Sep 09, 2021 at 09:04:28PM +0300, Lukas Prediger wrote:
> Hey Phil,
> 
> thanks for taking the time to look at this and give feedback.
> 

No problem, and great work :-)

> > Dear Lukas,
> >
> > Thank you for the patch, much appreciated and looks great. One very
> > minor thing though has jumped out at me after running checkpatch though:
> >
> > > --- 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
> > >   *******************************************************/
> > > @@ -295,6 +297,19 @@ struct cdrom_generic_command
> > >       };
> > >  };
> > >
> > > +/* This struct is used by CDROM_TIMED_MEDIA_CHANGE */
> > > +struct cdrom_timed_media_change_info
> > > +{
> >
> > This opening brace should be on the same line as the struct definition
> > as per current style rules.
> 
> I also noted that checkpatch reported this and that it is technically a style breach,
> however I kept the brace in the newline to be consistent with all the other
> cdrom ioctl structs defined above this in the same file for consistency.
> I have no strong feelings about this, though, so we could change this.
> 

I take your point, but my personal preference would be to not introduce
code that has new checkpatch errors or warnings, where possible.

Indeed, it may be worth correcting other code in the driver that would
trigger checkpatch violations, and this is something I will look at, provided
it makes sense and isn't too much churn for what are non-semantic changes to
a very stable driver. Many thanks.

Regards,
Phil

Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

[Lukas Prediger]

Hey Phil,

thanks for taking the time to look at this and give feedback.

> Dear Lukas,
>
> Thank you for the patch, much appreciated and looks great. One very
> minor thing though has jumped out at me after running checkpatch though:
>
> > --- 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
> >   *******************************************************/
> > @@ -295,6 +297,19 @@ struct cdrom_generic_command
> >       };
> >  };
> >
> > +/* This struct is used by CDROM_TIMED_MEDIA_CHANGE */
> > +struct cdrom_timed_media_change_info
> > +{
>
> This opening brace should be on the same line as the struct definition
> as per current style rules.

I also noted that checkpatch reported this and that it is technically a style breach,
however I kept the brace in the newline to be consistent with all the other
cdrom ioctl structs defined above this in the same file for consistency.
I have no strong feelings about this, though, so we could change this.

> I also got a checkpatch warning about ENOSYS being used as an error
> value, but I can see this usage is standard in the driver and not a
> problem so no issue with that.
> 
> I will review and test properly after work tomorrow (being new to the
> role I'd like to make sure I'm taking the proper time), but I have no
> doubt it will work fine. Assuming it does I will be happy to accept the
> patch with the above brace change. Thanks again.
> Regards,
> Phil

Best regards,
Lukas

Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

[Phillip Potter]

On Mon, Sep 06, 2021 at 02:11:31PM -0600, Jens Axboe wrote:
> On 8/29/21 8:37 AM, Lukas Prediger wrote:
> > 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.
> 
> This looks pretty good to me now. Adding Phillip to the CC, he's the new
> CDROM maintainer. Leaving the rest of the message below intact because
> of that.
> 
> 
> > Signed-off-by: Lukas Prediger <lumip@lumip.de>
> > ---
> > Second version based on your feedback for the first
> > attempt. Please let me know if further changes are required.
> > ---
> >  Documentation/cdrom/cdrom-standard.rst      | 11 ++++
> >  Documentation/userspace-api/ioctl/cdrom.rst |  3 ++
> >  drivers/cdrom/cdrom.c                       | 56 +++++++++++++++++++--
> >  include/linux/cdrom.h                       |  1 +
> >  include/uapi/linux/cdrom.h                  | 15 ++++++
> >  5 files changed, 82 insertions(+), 4 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_.
> > diff --git a/Documentation/userspace-api/ioctl/cdrom.rst b/Documentation/userspace-api/ioctl/cdrom.rst
> > index 3b4c0506de46..bac5bbf93ca0 100644
> > --- a/Documentation/userspace-api/ioctl/cdrom.rst
> > +++ b/Documentation/userspace-api/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 feb827eefd1a..a040a867f6a2 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)
> >  {
> > @@ -616,6 +622,7 @@ int register_cdrom(struct gendisk *disk, 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;
> > @@ -1421,8 +1428,7 @@ static int cdrom_select_disc(struct cdrom_device_info *cdi, int slot)
> >  		cdi->ops->check_events(cdi, 0, 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);
> >  	}
> >  
> > @@ -1455,7 +1461,7 @@ 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;
> >  
> > @@ -1521,7 +1527,7 @@ int media_changed(struct cdrom_device_info *cdi, int queue)
> >  	cdi->ioctl_events = 0;
> >  
> >  	if (changed) {
> > -		cdi->mc_flags = 0x3;    /* set bit on both queues */
> > +		signal_media_change(cdi);
> >  		ret |= 1;
> >  		cdi->media_written = 0;
> >  	}
> > @@ -2391,6 +2397,46 @@ 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.has_changed = ((tmp_info.last_media_change - cdi->last_media_change_ms) < 0);
> > +	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)
> >  {
> > @@ -3375,6 +3421,8 @@ 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 f48d0a31deae..7c10b564db29 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 6c34f6e2f1f7..9b17868667ed 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
> >   *******************************************************/
> > @@ -295,6 +297,19 @@ struct cdrom_generic_command
> >  	};
> >  };
> >  
> > +/* 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	has_changed;		/* Set to 1 by ioctl if last detected media
> > +					 * change was more recent than
> > +					 * last_media_change set by caller.
> > +					 */
> > +};
> > +
> >  /*
> >   * A CD-ROM physical sector size is 2048, 2052, 2056, 2324, 2332, 2336, 
> >   * 2340, or 2352 bytes long.  
> > 
> 
> 
> -- 
> Jens Axboe
> 

Dear Lukas,

I've now tested your patch, and it works perfectly for me. Also, I have
reviewed the code in more detail and it seems very good. For that
reason, I'm happy to send on to Jens for merging, provided the
previously mentioned struct bracket issue is sorted - as it is so
minor, I can do this for you when I send it on if you're happy with
that, to save you resubmitting? Let me know.

Many thanks for your work on this, much appreciated.

Regards,
Phil

Re: [PATCH v2] drivers/cdrom: improved ioctl for media change detection

[Phillip Potter]

On Mon, Sep 06, 2021 at 02:11:31PM -0600, Jens Axboe wrote:
> On 8/29/21 8:37 AM, Lukas Prediger wrote:
> > 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.
> 
> This looks pretty good to me now. Adding Phillip to the CC, he's the new
> CDROM maintainer. Leaving the rest of the message below intact because
> of that.
> 
> >
...
> >

Dear Lukas,

Thank you for the patch, much appreciated and looks great. One very
minor thing though has jumped out at me after running checkpatch though:

> > --- 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
> >   *******************************************************/
> > @@ -295,6 +297,19 @@ struct cdrom_generic_command
> >       };
> >  };
> >
> > +/* This struct is used by CDROM_TIMED_MEDIA_CHANGE */
> > +struct cdrom_timed_media_change_info
> > +{

This opening brace should be on the same line as the struct definition
as per current style rules.

I also got a checkpatch warning about ENOSYS being used as an error
value, but I can see this usage is standard in the driver and not a
problem so no issue with that.

I will review and test properly after work tomorrow (being new to the
role I'd like to make sure I'm taking the proper time), but I have no
doubt it will work fine. Assuming it does I will be happy to accept the
patch with the above brace change. Thanks again.

> > +	__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	has_changed;		/* Set to 1 by ioctl if last detected media
> > +					 * change was more recent than
> > +					 * last_media_change set by caller.
> > +					 */
> > +};
> > +
> >  /*
> >   * A CD-ROM physical sector size is 2048, 2052, 2056, 2324, 2332, 2336, 
> >   * 2340, or 2352 bytes long.  
> > 

Regards,
Phil