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

[Coiby Xu <coiby.xu@gmail.com>]

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,
>>
>> $ make C=2 drivers/staging/greybus/
>> drivers/staging/greybus/audio_topology.c:185:24: warning: cast to restricted snd_ctl_elem_type_t
>> drivers/staging/greybus/audio_topology.c:679:14: warning: restricted snd_ctl_elem_iface_t degrades to integer
>> drivers/staging/greybus/audio_topology.c:906:14: warning: restricted snd_ctl_elem_iface_t degrades to integer
>>
>> Signed-off-by: Coiby Xu <coiby.xu@gmail.com>
>> ---
>>  drivers/staging/greybus/audio_topology.c | 2 +-
>>  include/uapi/sound/asound.h              | 4 ++--
>>  2 files changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/staging/greybus/audio_topology.c b/drivers/staging/greybus/audio_topology.c
>> index 56bf1a4f95ad..f6023ff390c2 100644
>> --- a/drivers/staging/greybus/audio_topology.c
>> +++ b/drivers/staging/greybus/audio_topology.c
>> @@ -182,7 +182,7 @@ static int gbcodec_mixer_ctl_info(struct snd_kcontrol *kcontrol,
>>  	/* update uinfo */
>>  	uinfo->access = data->access;
>>  	uinfo->count = data->vcount;
>> -	uinfo->type = (snd_ctl_elem_type_t)info->type;
>> +	uinfo->type = info->type;
>>
>>  	switch (info->type) {
>>  	case GB_AUDIO_CTL_ELEM_TYPE_BOOLEAN:
>> 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.

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.

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,

$ 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