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

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Date:   Thu, 9 Sep 2021 18:25:35 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Lukas Prediger <lumip@...ip.de>
Cc:     axboe@...nel.dk, hch@...radead.org, linux-kernel@...r.kernel.org,
        phil@...lpotter.co.uk
Subject: Re: [PATCH v2] drivers/cdrom: improved ioctl for media change
 detection

On 9/9/21 11:05 AM, Lukas Prediger wrote:
>>> 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/
> 
> 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 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