linux-kernel - Re: [PATCH 3/3] [PATCH] staging: greybus: __u8 is sufficient for snd_ctl_elem_type_t and snd_ctl_elem_iface

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Date:   Sat, 26 Sep 2020 13:09:58 +0800
From:   Coiby Xu <coiby.xu@...il.com>
To:     Alex Elder <elder@...aro.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devel@...verdev.osuosl.org,
        "moderated list:SOUND" <alsa-devel@...a-project.org>,
        Vaibhav Agarwal <vaibhav.sr@...il.com>,
        Mark Greer <mgreer@...malcreek.com>,
        Takashi Iwai <tiwai@...e.com>, Johan Hovold <johan@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        "moderated list:GREYBUS SUBSYSTEM" <greybus-dev@...ts.linaro.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] [PATCH] staging: greybus: __u8 is sufficient for
 snd_ctl_elem_type_t and snd_ctl_elem_iface_t

On Fri, Sep 25, 2020 at 10:57:27AM -0500, Alex Elder wrote:
>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.

Thank you for sharing me about the byte order of user space API which
prompts me to examine where "info->type" comes from,

	uinfo->type = (snd_ctl_elem_type_t)info->type;

Eventually it comes from the parsed topology data which is obtained via
gb_audio_gb_get_topology(gbmodule->mgmt_connection, &topology). But
since (struct gb_audio_ctl_elem_info*)->type has __u8 type, there is no
endianness concern. Then based on the principles of userspace API
shouldn't be modified and greybus is operating system independent, your
previous suggestion to use __force which means "I know what I'm doing"
is actually a good idea,

         uinfo->type = (__force snd_ctl_elem_type_t)info->type;


>>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
>

--
Best regards,
Coiby