Re: [PATCH 3/3] [PATCH] staging: greybus: __u8 is sufficient for snd_ctl_elem_type_t and snd_ctl_elem_iface_t

[Alex Elder]

On 9/25/20 9:07 AM, Coiby Xu wrote:
> On Thu, Sep 24, 2020 at 01:00:57PM +0200, Greg Kroah-Hartman wrote:
> > On Thu, Sep 24, 2020 at 06:20:39PM +0800, Coiby Xu wrote:
> > > Use __8 to replace int and remove the unnecessary __bitwise type 
> > > attribute.
> > >
> > > Found by sparse,

. . .

> > > diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
> > > index 535a7229e1d9..8e71a95644ab 100644
> > > --- a/include/uapi/sound/asound.h
> > > +++ b/include/uapi/sound/asound.h
> > > @@ -950,7 +950,7 @@ struct snd_ctl_card_info {
> > >      unsigned char components[128];    /* card components / fine 
> > > identification, delimited with one space (AC97 etc..) */
> > >  };
> > >
> > > -typedef int __bitwise snd_ctl_elem_type_t;
> > > +typedef __u8 snd_ctl_elem_type_t;
> > >  #define    SNDRV_CTL_ELEM_TYPE_NONE    ((__force 
> > > snd_ctl_elem_type_t) 0) /* invalid */
> > >  #define    SNDRV_CTL_ELEM_TYPE_BOOLEAN    ((__force 
> > > snd_ctl_elem_type_t) 1) /* boolean type */
> > >  #define    SNDRV_CTL_ELEM_TYPE_INTEGER    ((__force 
> > > snd_ctl_elem_type_t) 2) /* integer type */
> > > @@ -960,7 +960,7 @@ typedef int __bitwise snd_ctl_elem_type_t;
> > >  #define    SNDRV_CTL_ELEM_TYPE_INTEGER64    ((__force 
> > > snd_ctl_elem_type_t) 6) /* 64-bit integer type */
> > >  #define    SNDRV_CTL_ELEM_TYPE_LAST    SNDRV_CTL_ELEM_TYPE_INTEGER64
> > >
> > > -typedef int __bitwise snd_ctl_elem_iface_t;
> > > +typedef __u8 snd_ctl_elem_iface_t;
> > >  #define    SNDRV_CTL_ELEM_IFACE_CARD    ((__force 
> > > snd_ctl_elem_iface_t) 0) /* global control */
> > >  #define    SNDRV_CTL_ELEM_IFACE_HWDEP    ((__force 
> > > snd_ctl_elem_iface_t) 1) /* hardware dependent device */
> > >  #define    SNDRV_CTL_ELEM_IFACE_MIXER    ((__force 
> > > snd_ctl_elem_iface_t) 2) /* virtual mixer device */
> >
> > I can't take a uapi sound header file patch along with a driver change,
> > these need to be independant.
>
> Thank you and Alex for reminding me this is a change of public header!
> > And this is a userspace api, you just changed the size of an externally
> > facing variable, are you _SURE_ that is ok to do?
>
> The reasons I make this change are, 1) using int to represent 7 enumeration
> values seems to be overkill; 2) using one type could avoid worries
> about byte order.

(1) might be a valid suggestion, but the file you suggest
changing is part of the Linux user space API, which you
basically can't change.

I'm fairly certain the user space API is defined to have
CPU-local byte order (unless specified explicitly otherwise)
so that is not a concern.

> I examine one userspace example (libalsa-intf/alsa_mixer.c [1]). This
> change won't cause compiling breakage. But I don't the experience to
> imagine any other bad consequence.

As a rule, once the user space API has been established, it
stays with us forever.  You can add to it, but modifying
(or removing) an existing interface is essentially forbidden.

> Another way to avoid userspace API change is to let
> "struct gb_audio_ctl_elem_info" use snd_ctl_elem_iface_t type which may
> be more elegant than using "__force" as suggested by Alex,

The Greybus definitions were explicitly defined to be
operating system independent.  For that reason there are
(admittedly cumbersome) definitions of certain types and
values that essentially mimic those defined by Linux
exactly  That way Linux (or another system using Greybus)
can change its internal values without changing the
Greybus API definition.  (This addresses the concern you
mention below.)

What you are suggesting is not consistent with that
principle.

					-Alex


> $ git diff
> diff --git a/include/linux/greybus/greybus_protocols.h 
> b/include/linux/greybus/greybus_protocols.h
> index aeb8f9243545..7f6753ec7ef7 100644
> --- a/include/linux/greybus/greybus_protocols.h
> +++ b/include/linux/greybus/greybus_protocols.h
> @@ -8,6 +8,7 @@
>   #define __GREYBUS_PROTOCOLS_H
>
>   #include <linux/types.h>
> +#include <sound/asound.h>
>
>   /* Fixed IDs for control/svc protocols */
>
> @@ -1997,7 +1998,7 @@ struct gb_audio_enumerated {
>   } __packed;
>
>   struct gb_audio_ctl_elem_info { /* See snd_ctl_elem_info in Linux 
> source */
> -       __u8            type;           /* GB_AUDIO_CTL_ELEM_TYPE_* */
> +       snd_ctl_elem_type_t             type;           /* 
> GB_AUDIO_CTL_ELEM_TYPE_* */
>          __le16          dimen[4];
>          union {
>                  struct gb_audio_integer         integer
>
> My only concern is if greybus authors have any special reason to not 
> include
> sound/asound.h in the first place and re-define things like 
> SNDRV_CTL_ELEM_TYPE_*,
>
> /* See SNDRV_CTL_ELEM_TYPE_* in Linux source */
> #define GB_AUDIO_CTL_ELEM_TYPE_BOOLEAN        0x01
> #define GB_AUDIO_CTL_ELEM_TYPE_INTEGER        0x02
>
> /* See SNDRV_CTL_ELEM_IFACE_* in Linux source */
> #define GB_AUDIO_CTL_ELEM_IFACE_CARD        0x00
> ...
>
> /* SNDRV_CTL_ELEM_ACCESS_* in Linux source */
> #define GB_AUDIO_ACCESS_READ            BIT(0)
> ...
>
> [1] 
> https://gitlab.com/Codeaurora/platform_hardware_qcom_audio/-/blob/aee6032826ec7c8b6edda404422fda0ef40ec2db/libalsa-intf/alsa_mixer.c 
>
>   alsa_mixer.c
> > thanks,
> >
> > greg k-h
>
> --
> Best regards,
> Coiby