[PATCH] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[PATCH] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Nathan Chancellor]

Clang warns:

sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
unused [-Wunused-value]
                        for (i = 0; i < count; i++, (u16 *)buf++)
                                                    ^      ~~~~~
1 warning generated.

It appears the intention was to cast the void pointer to a u16 pointer
so that the data could be iterated through like an array of u16 values.
However, the cast happens after the increment because a cast is an
rvalue, whereas the post-increment operator only works on lvalues, so
the loop does not iterate as expected.

Replace the post-increment shorthand with the full expression so the
cast can be added in the right place and the look works as expected.

Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
Link: https://github.com/ClangBuiltLinux/linux/issues/1408
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 sound/usb/mixer_scarlett_gen2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
index fcba682cd422..c20c7f1ddc50 100644
--- a/sound/usb/mixer_scarlett_gen2.c
+++ b/sound/usb/mixer_scarlett_gen2.c
@@ -1186,7 +1186,7 @@ static int scarlett2_usb_get_config(
 		if (err < 0)
 			return err;
 		if (size == 2)
-			for (i = 0; i < count; i++, (u16 *)buf++)
+			for (i = 0; i < count; i++, buf = (u16 *)buf + 1)
 				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
 		return 0;
 	}

base-commit: 5c89c2c7fbfa9124dd521c375b9c82b9ed75bc28
-- 
2.32.0.93.g670b81a890

Re: [PATCH] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Geoffrey D. Bennett]

On Thu, Jun 24, 2021 at 02:20:48PM -0700, Nathan Chancellor wrote:
> Clang warns:
> 
> sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
> unused [-Wunused-value]
>                         for (i = 0; i < count; i++, (u16 *)buf++)
>                                                     ^      ~~~~~
> 1 warning generated.
> 
> It appears the intention was to cast the void pointer to a u16 pointer
> so that the data could be iterated through like an array of u16 values.
> However, the cast happens after the increment because a cast is an
> rvalue, whereas the post-increment operator only works on lvalues, so
> the loop does not iterate as expected.
> 
> Replace the post-increment shorthand with the full expression so the
> cast can be added in the right place and the look works as expected.

look -> loop

> Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1408
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  sound/usb/mixer_scarlett_gen2.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
> index fcba682cd422..c20c7f1ddc50 100644
> --- a/sound/usb/mixer_scarlett_gen2.c
> +++ b/sound/usb/mixer_scarlett_gen2.c
> @@ -1186,7 +1186,7 @@ static int scarlett2_usb_get_config(
>  		if (err < 0)
>  			return err;
>  		if (size == 2)
> -			for (i = 0; i < count; i++, (u16 *)buf++)
> +			for (i = 0; i < count; i++, buf = (u16 *)buf + 1)
>  				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
>  		return 0;
>  	}

Thanks Nathan!

FYI: although incorrect, this caused no bug as there is not yet any
case where count > 1.

Acked-by: Geoffrey D. Bennett <g@b4.vu>

Re: [PATCH] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Takashi Iwai]

On Thu, 24 Jun 2021 23:20:48 +0200,
Nathan Chancellor wrote:
> 
> Clang warns:
> 
> sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
> unused [-Wunused-value]
>                         for (i = 0; i < count; i++, (u16 *)buf++)
>                                                     ^      ~~~~~
> 1 warning generated.
> 
> It appears the intention was to cast the void pointer to a u16 pointer
> so that the data could be iterated through like an array of u16 values.
> However, the cast happens after the increment because a cast is an
> rvalue, whereas the post-increment operator only works on lvalues, so
> the loop does not iterate as expected.
> 
> Replace the post-increment shorthand with the full expression so the
> cast can be added in the right place and the look works as expected.
> 
> Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1408
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  sound/usb/mixer_scarlett_gen2.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
> index fcba682cd422..c20c7f1ddc50 100644
> --- a/sound/usb/mixer_scarlett_gen2.c
> +++ b/sound/usb/mixer_scarlett_gen2.c
> @@ -1186,7 +1186,7 @@ static int scarlett2_usb_get_config(
>  		if (err < 0)
>  			return err;
>  		if (size == 2)
> -			for (i = 0; i < count; i++, (u16 *)buf++)
> +			for (i = 0; i < count; i++, buf = (u16 *)buf + 1)
>  				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);

That's still too error-prone.

Could you rather introduce another variable of u16 * type, and use it
instead?  Ditto for u8 access for the code after that, too.


thanks,

Takashi

[PATCH v2] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Nathan Chancellor]

Clang warns:

sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
unused [-Wunused-value]
                        for (i = 0; i < count; i++, (u16 *)buf++)
                                                    ^      ~~~~~
1 warning generated.

It appears the intention was to cast the void pointer to a u16 pointer
so that the data could be iterated through like an array of u16 values.
However, the cast happens after the increment because a cast is an
rvalue, whereas the post-increment operator only works on lvalues, so
the loop does not iterate as expected. This is not a bug in practice
because count is not greater than one at the moment but this could
change in the future so this should be fixed.

Replace the cast with a temporary variable of the proper type, which is
less error prone and fixes the iteration. Do the same thing for the
'u8 *' below this if block.

Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
Link: https://github.com/ClangBuiltLinux/linux/issues/1408
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---

v1 -> v2:

* Use temporary variables of proper type rather than casting, as
  requested by Takashi. I did not include Geoffrey's ack for this
  reason.

* Mention that there is not a bug at the moment per Geoffrey's comment.

 sound/usb/mixer_scarlett_gen2.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
index fcba682cd422..b13903bed330 100644
--- a/sound/usb/mixer_scarlett_gen2.c
+++ b/sound/usb/mixer_scarlett_gen2.c
@@ -1177,17 +1177,22 @@ static int scarlett2_usb_get_config(
 	const struct scarlett2_config *config_item =
 		&scarlett2_config_items[info->has_mixer][config_item_num];
 	int size, err, i;
+	u8 *buf_8;
 	u8 value;
 
 	/* For byte-sized parameters, retrieve directly into buf */
 	if (config_item->size >= 8) {
+		u16 *buf_16;
+
 		size = config_item->size / 8 * count;
 		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
 		if (err < 0)
 			return err;
-		if (size == 2)
-			for (i = 0; i < count; i++, (u16 *)buf++)
-				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
+		if (size == 2) {
+			buf_16 = buf;
+			for (i = 0; i < count; i++, buf_16++)
+				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
+		}
 		return 0;
 	}
 
@@ -1197,8 +1202,9 @@ static int scarlett2_usb_get_config(
 		return err;
 
 	/* then unpack from value into buf[] */
+	buf_8 = buf;
 	for (i = 0; i < 8 && i < count; i++, value >>= 1)
-		*(u8 *)buf++ = value & 1;
+		*buf_8++ = value & 1;
 
 	return 0;
 }

base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
-- 
2.32.0.93.g670b81a890

Re: [PATCH v2] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Geoffrey D. Bennett]

On Fri, Jun 25, 2021 at 10:54:19AM -0700, Nathan Chancellor wrote:
> Clang warns:
> 
> sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
> unused [-Wunused-value]
>                         for (i = 0; i < count; i++, (u16 *)buf++)
>                                                     ^      ~~~~~
> 1 warning generated.
> 
> It appears the intention was to cast the void pointer to a u16 pointer
> so that the data could be iterated through like an array of u16 values.
> However, the cast happens after the increment because a cast is an
> rvalue, whereas the post-increment operator only works on lvalues, so
> the loop does not iterate as expected. This is not a bug in practice
> because count is not greater than one at the moment but this could
> change in the future so this should be fixed.
> 
> Replace the cast with a temporary variable of the proper type, which is
> less error prone and fixes the iteration. Do the same thing for the
> 'u8 *' below this if block.
> 
> Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1408
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> 
> v1 -> v2:
> 
> * Use temporary variables of proper type rather than casting, as
>   requested by Takashi. I did not include Geoffrey's ack for this
>   reason.
> 
> * Mention that there is not a bug at the moment per Geoffrey's comment.
> 
>  sound/usb/mixer_scarlett_gen2.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
> index fcba682cd422..b13903bed330 100644
> --- a/sound/usb/mixer_scarlett_gen2.c
> +++ b/sound/usb/mixer_scarlett_gen2.c
> @@ -1177,17 +1177,22 @@ static int scarlett2_usb_get_config(
>  	const struct scarlett2_config *config_item =
>  		&scarlett2_config_items[info->has_mixer][config_item_num];
>  	int size, err, i;
> +	u8 *buf_8;
>  	u8 value;
>  
>  	/* For byte-sized parameters, retrieve directly into buf */
>  	if (config_item->size >= 8) {
> +		u16 *buf_16;
> +

I would prefer that the u16 *buf_16 declaration above be removed from
there...

>  		size = config_item->size / 8 * count;
>  		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
>  		if (err < 0)
>  			return err;
> -		if (size == 2)
> -			for (i = 0; i < count; i++, (u16 *)buf++)
> -				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
> +		if (size == 2) {
> +			buf_16 = buf;

...and combined with the assignment here, like: u16 *buf_16 = buf;

Regardless:

Acked-by: Geoffrey D. Bennett <g@b4.vu>

And, thanks again!

> +			for (i = 0; i < count; i++, buf_16++)
> +				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
> +		}
>  		return 0;
>  	}
>  
> @@ -1197,8 +1202,9 @@ static int scarlett2_usb_get_config(
>  		return err;
>  
>  	/* then unpack from value into buf[] */
> +	buf_8 = buf;
>  	for (i = 0; i < 8 && i < count; i++, value >>= 1)
> -		*(u8 *)buf++ = value & 1;
> +		*buf_8++ = value & 1;
>  
>  	return 0;
>  }
> 
> base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
> -- 
> 2.32.0.93.g670b81a890
> 

[PATCH v3] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Nathan Chancellor]

Clang warns:

sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
unused [-Wunused-value]
                        for (i = 0; i < count; i++, (u16 *)buf++)
                                                    ^      ~~~~~
1 warning generated.

It appears the intention was to cast the void pointer to a u16 pointer
so that the data could be iterated through like an array of u16 values.
However, the cast happens after the increment because a cast is an
rvalue, whereas the post-increment operator only works on lvalues, so
the loop does not iterate as expected.

Replace the cast with a temporary variable of the proper type, which is
less error prone and fixes the iteration. Do the same thing for the
'u8 *' below this if block.

Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
Link: https://github.com/ClangBuiltLinux/linux/issues/1408
Acked-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---

v1 -> v2:

* Use temporary variables of proper type rather than casting, as
  requested by Takashi.

* Mention that there is not a bug at the moment per Geoffrey's comment.

v2 -> v3:

* Restrict scope of buf_16 more, as requested by Geoffrey.

* Add Geoffrey's ack.

 sound/usb/mixer_scarlett_gen2.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
index fcba682cd422..07ba0ef884a2 100644
--- a/sound/usb/mixer_scarlett_gen2.c
+++ b/sound/usb/mixer_scarlett_gen2.c
@@ -1177,17 +1177,22 @@ static int scarlett2_usb_get_config(
 	const struct scarlett2_config *config_item =
 		&scarlett2_config_items[info->has_mixer][config_item_num];
 	int size, err, i;
+	u8 *buf_8;
 	u8 value;
 
 	/* For byte-sized parameters, retrieve directly into buf */
 	if (config_item->size >= 8) {
+
 		size = config_item->size / 8 * count;
 		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
 		if (err < 0)
 			return err;
-		if (size == 2)
-			for (i = 0; i < count; i++, (u16 *)buf++)
-				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
+		if (size == 2) {
+			u16 *buf_16 = buf;
+
+			for (i = 0; i < count; i++, buf_16++)
+				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
+		}
 		return 0;
 	}
 
@@ -1197,8 +1202,9 @@ static int scarlett2_usb_get_config(
 		return err;
 
 	/* then unpack from value into buf[] */
+	buf_8 = buf;
 	for (i = 0; i < 8 && i < count; i++, value >>= 1)
-		*(u8 *)buf++ = value & 1;
+		*buf_8++ = value & 1;
 
 	return 0;
 }

base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
-- 
2.32.0.93.g670b81a890

[PATCH v4] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Nathan Chancellor]

Clang warns:

sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
unused [-Wunused-value]
                        for (i = 0; i < count; i++, (u16 *)buf++)
                                                    ^      ~~~~~
1 warning generated.

It appears the intention was to cast the void pointer to a u16 pointer
so that the data could be iterated through like an array of u16 values.
However, the cast happens after the increment because a cast is an
rvalue, whereas the post-increment operator only works on lvalues, so
the loop does not iterate as expected.

Replace the cast with a temporary variable of the proper type, which is
less error prone and fixes the iteration. Do the same thing for the
'u8 *' below this if block.

Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
Link: https://github.com/ClangBuiltLinux/linux/issues/1408
Acked-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---

v1 -> v2:

* Use temporary variables of proper type rather than casting, as
  requested by Takashi.

* Mention that there is not a bug at the moment per Geoffrey's comment.

v2 -> v3:

* Restrict scope of buf_16 more, as requested by Geoffrey.

* Add Geoffrey's ack.

v3 -> v4:

* Fix stray newline added below

  if (config_item->size >= 8) {

  leftover from buf_16's declaration.

 sound/usb/mixer_scarlett_gen2.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
index fcba682cd422..161d832cafef 100644
--- a/sound/usb/mixer_scarlett_gen2.c
+++ b/sound/usb/mixer_scarlett_gen2.c
@@ -1177,6 +1177,7 @@ static int scarlett2_usb_get_config(
 	const struct scarlett2_config *config_item =
 		&scarlett2_config_items[info->has_mixer][config_item_num];
 	int size, err, i;
+	u8 *buf_8;
 	u8 value;
 
 	/* For byte-sized parameters, retrieve directly into buf */
@@ -1185,9 +1186,12 @@ static int scarlett2_usb_get_config(
 		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
 		if (err < 0)
 			return err;
-		if (size == 2)
-			for (i = 0; i < count; i++, (u16 *)buf++)
-				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
+		if (size == 2) {
+			u16 *buf_16 = buf;
+
+			for (i = 0; i < count; i++, buf_16++)
+				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
+		}
 		return 0;
 	}
 
@@ -1197,8 +1201,9 @@ static int scarlett2_usb_get_config(
 		return err;
 
 	/* then unpack from value into buf[] */
+	buf_8 = buf;
 	for (i = 0; i < 8 && i < count; i++, value >>= 1)
-		*(u8 *)buf++ = value & 1;
+		*buf_8++ = value & 1;
 
 	return 0;
 }

base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
-- 
2.32.0.93.g670b81a890

Re: [PATCH v4] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Geoffrey D. Bennett]

On Fri, Jun 25, 2021 at 01:11:51PM -0700, Nathan Chancellor wrote:
> Clang warns:
> 
> sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
> unused [-Wunused-value]
>                         for (i = 0; i < count; i++, (u16 *)buf++)
>                                                     ^      ~~~~~
> 1 warning generated.
> 
> It appears the intention was to cast the void pointer to a u16 pointer
> so that the data could be iterated through like an array of u16 values.
> However, the cast happens after the increment because a cast is an
> rvalue, whereas the post-increment operator only works on lvalues, so
> the loop does not iterate as expected.

Your note about no bug which was added in v2 went missing in v3:

> > the loop does not iterate as expected. This is not a bug in practice
> > because count is not greater than one at the moment but this could
> > change in the future so this should be fixed.

> Replace the cast with a temporary variable of the proper type, which is
> less error prone and fixes the iteration. Do the same thing for the
> 'u8 *' below this if block.
> 
> Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1408
> Acked-by: Geoffrey D. Bennett <g@b4.vu>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> 
> v1 -> v2:
> 
> * Use temporary variables of proper type rather than casting, as
>   requested by Takashi.
> 
> * Mention that there is not a bug at the moment per Geoffrey's comment.
> 
> v2 -> v3:
> 
> * Restrict scope of buf_16 more, as requested by Geoffrey.
> 
> * Add Geoffrey's ack.
> 
> v3 -> v4:
> 
> * Fix stray newline added below
> 
>   if (config_item->size >= 8) {
> 
>   leftover from buf_16's declaration.
> 
>  sound/usb/mixer_scarlett_gen2.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
> index fcba682cd422..161d832cafef 100644
> --- a/sound/usb/mixer_scarlett_gen2.c
> +++ b/sound/usb/mixer_scarlett_gen2.c
> @@ -1177,6 +1177,7 @@ static int scarlett2_usb_get_config(
>  	const struct scarlett2_config *config_item =
>  		&scarlett2_config_items[info->has_mixer][config_item_num];
>  	int size, err, i;
> +	u8 *buf_8;
>  	u8 value;
>  
>  	/* For byte-sized parameters, retrieve directly into buf */
> @@ -1185,9 +1186,12 @@ static int scarlett2_usb_get_config(
>  		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
>  		if (err < 0)
>  			return err;
> -		if (size == 2)
> -			for (i = 0; i < count; i++, (u16 *)buf++)
> -				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
> +		if (size == 2) {
> +			u16 *buf_16 = buf;
> +
> +			for (i = 0; i < count; i++, buf_16++)
> +				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
> +		}
>  		return 0;
>  	}
>  
> @@ -1197,8 +1201,9 @@ static int scarlett2_usb_get_config(
>  		return err;
>  
>  	/* then unpack from value into buf[] */
> +	buf_8 = buf;
>  	for (i = 0; i < 8 && i < count; i++, value >>= 1)
> -		*(u8 *)buf++ = value & 1;
> +		*buf_8++ = value & 1;
>  
>  	return 0;
>  }
> 
> base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
> -- 
> 2.32.0.93.g670b81a890
> 

[PATCH v5] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Nathan Chancellor]

Clang warns:

sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
unused [-Wunused-value]
                        for (i = 0; i < count; i++, (u16 *)buf++)
                                                    ^      ~~~~~
1 warning generated.

It appears the intention was to cast the void pointer to a u16 pointer
so that the data could be iterated through like an array of u16 values.
However, the cast happens after the increment because a cast is an
rvalue, whereas the post-increment operator only works on lvalues, so
the loop does not iterate as expected. This is not a bug in practice
because count is not greater than one at the moment but this could
change in the future so this should be fixed.

Replace the cast with a temporary variable of the proper type, which is
less error prone and fixes the iteration. Do the same thing for the
'u8 *' below this if block.

Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
Link: https://github.com/ClangBuiltLinux/linux/issues/1408
Acked-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---

v1 -> v2:

* Use temporary variables of proper type rather than casting, as
  requested by Takashi.

* Mention that there is not a bug at the moment per Geoffrey's comment.

v2 -> v3:

* Restrict scope of buf_16 more, as requested by Geoffrey.

* Add Geoffrey's ack.

v3 -> v4:

* Fix stray newline added below

  if (config_item->size >= 8) {

  leftover from buf_16's declaration.

v4 -> v5 (or how many times does it take Nathan to get a patch right):

* Re-add note about no bug that was dropped in v3 by accident, as
  noticed by Geoffrey. My apologies for the multiple revisions.

 sound/usb/mixer_scarlett_gen2.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
index fcba682cd422..161d832cafef 100644
--- a/sound/usb/mixer_scarlett_gen2.c
+++ b/sound/usb/mixer_scarlett_gen2.c
@@ -1177,6 +1177,7 @@ static int scarlett2_usb_get_config(
 	const struct scarlett2_config *config_item =
 		&scarlett2_config_items[info->has_mixer][config_item_num];
 	int size, err, i;
+	u8 *buf_8;
 	u8 value;
 
 	/* For byte-sized parameters, retrieve directly into buf */
@@ -1185,9 +1186,12 @@ static int scarlett2_usb_get_config(
 		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
 		if (err < 0)
 			return err;
-		if (size == 2)
-			for (i = 0; i < count; i++, (u16 *)buf++)
-				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
+		if (size == 2) {
+			u16 *buf_16 = buf;
+
+			for (i = 0; i < count; i++, buf_16++)
+				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
+		}
 		return 0;
 	}
 
@@ -1197,8 +1201,9 @@ static int scarlett2_usb_get_config(
 		return err;
 
 	/* then unpack from value into buf[] */
+	buf_8 = buf;
 	for (i = 0; i < 8 && i < count; i++, value >>= 1)
-		*(u8 *)buf++ = value & 1;
+		*buf_8++ = value & 1;
 
 	return 0;
 }

base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
-- 
2.32.0.93.g670b81a890

Re: [PATCH v5] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Takashi Iwai]

On Sun, 27 Jun 2021 07:12:03 +0200,
Nathan Chancellor wrote:
> 
> Clang warns:
> 
> sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
> unused [-Wunused-value]
>                         for (i = 0; i < count; i++, (u16 *)buf++)
>                                                     ^      ~~~~~
> 1 warning generated.
> 
> It appears the intention was to cast the void pointer to a u16 pointer
> so that the data could be iterated through like an array of u16 values.
> However, the cast happens after the increment because a cast is an
> rvalue, whereas the post-increment operator only works on lvalues, so
> the loop does not iterate as expected. This is not a bug in practice
> because count is not greater than one at the moment but this could
> change in the future so this should be fixed.
> 
> Replace the cast with a temporary variable of the proper type, which is
> less error prone and fixes the iteration. Do the same thing for the
> 'u8 *' below this if block.
> 
> Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1408
> Acked-by: Geoffrey D. Bennett <g@b4.vu>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> 
> v1 -> v2:
> 
> * Use temporary variables of proper type rather than casting, as
>   requested by Takashi.
> 
> * Mention that there is not a bug at the moment per Geoffrey's comment.
> 
> v2 -> v3:
> 
> * Restrict scope of buf_16 more, as requested by Geoffrey.
> 
> * Add Geoffrey's ack.
> 
> v3 -> v4:
> 
> * Fix stray newline added below
> 
>   if (config_item->size >= 8) {
> 
>   leftover from buf_16's declaration.
> 
> v4 -> v5 (or how many times does it take Nathan to get a patch right):
> 
> * Re-add note about no bug that was dropped in v3 by accident, as
>   noticed by Geoffrey. My apologies for the multiple revisions.

Thanks, applied now.


Takashi

Re: [PATCH v2] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Nathan Chancellor]

On Sat, Jun 26, 2021 at 04:13:42AM +0930, Geoffrey D. Bennett wrote:
> On Fri, Jun 25, 2021 at 10:54:19AM -0700, Nathan Chancellor wrote:
> > Clang warns:
> > 
> > sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
> > unused [-Wunused-value]
> >                         for (i = 0; i < count; i++, (u16 *)buf++)
> >                                                     ^      ~~~~~
> > 1 warning generated.
> > 
> > It appears the intention was to cast the void pointer to a u16 pointer
> > so that the data could be iterated through like an array of u16 values.
> > However, the cast happens after the increment because a cast is an
> > rvalue, whereas the post-increment operator only works on lvalues, so
> > the loop does not iterate as expected. This is not a bug in practice
> > because count is not greater than one at the moment but this could
> > change in the future so this should be fixed.
> > 
> > Replace the cast with a temporary variable of the proper type, which is
> > less error prone and fixes the iteration. Do the same thing for the
> > 'u8 *' below this if block.
> > 
> > Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1408
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > ---
> > 
> > v1 -> v2:
> > 
> > * Use temporary variables of proper type rather than casting, as
> >   requested by Takashi. I did not include Geoffrey's ack for this
> >   reason.
> > 
> > * Mention that there is not a bug at the moment per Geoffrey's comment.
> > 
> >  sound/usb/mixer_scarlett_gen2.c | 14 ++++++++++----
> >  1 file changed, 10 insertions(+), 4 deletions(-)
> > 
> > diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
> > index fcba682cd422..b13903bed330 100644
> > --- a/sound/usb/mixer_scarlett_gen2.c
> > +++ b/sound/usb/mixer_scarlett_gen2.c
> > @@ -1177,17 +1177,22 @@ static int scarlett2_usb_get_config(
> >  	const struct scarlett2_config *config_item =
> >  		&scarlett2_config_items[info->has_mixer][config_item_num];
> >  	int size, err, i;
> > +	u8 *buf_8;
> >  	u8 value;
> >  
> >  	/* For byte-sized parameters, retrieve directly into buf */
> >  	if (config_item->size >= 8) {
> > +		u16 *buf_16;
> > +
> 
> I would prefer that the u16 *buf_16 declaration above be removed from
> there...
> 
> >  		size = config_item->size / 8 * count;
> >  		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
> >  		if (err < 0)
> >  			return err;
> > -		if (size == 2)
> > -			for (i = 0; i < count; i++, (u16 *)buf++)
> > -				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
> > +		if (size == 2) {
> > +			buf_16 = buf;
> 
> ...and combined with the assignment here, like: u16 *buf_16 = buf;

Thanks for pointing it out, I was not paying enough attention to realize
that the scope could be reduced. v3 sent with your Ack added, thank you
for taking a look in a quick manner!

Cheers,
Nathan

> Regardless:
> 
> Acked-by: Geoffrey D. Bennett <g@b4.vu>
> 
> And, thanks again!
> 
> > +			for (i = 0; i < count; i++, buf_16++)
> > +				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
> > +		}
> >  		return 0;
> >  	}
> >  
> > @@ -1197,8 +1202,9 @@ static int scarlett2_usb_get_config(
> >  		return err;
> >  
> >  	/* then unpack from value into buf[] */
> > +	buf_8 = buf;
> >  	for (i = 0; i < 8 && i < count; i++, value >>= 1)
> > -		*(u8 *)buf++ = value & 1;
> > +		*buf_8++ = value & 1;
> >  
> >  	return 0;
> >  }
> > 
> > base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
> > -- 
> > 2.32.0.93.g670b81a890
> >