lkml.org 
[lkml]   [2021]   [Sep]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v2] drivers/cdrom: improved ioctl for media change detection
From
Date
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

\
 
 \ /
  Last update: 2021-09-10 03:26    [W:0.106 / U:0.184 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site