linux-kernel - Re: Question about hwmon_attr_show

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Message-ID: <14C0030C-C764-45AE-BA2D-881051004490@gmx.de>
Date:   Tue, 07 Mar 2017 10:14:21 +0100
From:   Peter Huewe <peterhuewe@....de>
To:     Jean Delvare <jdelvare@...e.de>, Guenter Roeck <linux@...ck-us.net>
CC:     linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Question about hwmon_attr_show_string



Am 7. März 2017 10:08:46 MEZ schrieb Jean Delvare <jdelvare@...e.de>:
>Hi Guenter,
>
>On Mon, 6 Mar 2017 15:47:55 -0800, Guenter Roeck wrote:
>> On Mon, Mar 06, 2017 at 09:48:35PM +0100, Peter Hüwe wrote:
>> > Hi Guenter,
>> > 
>> > I was wondering whether there was a particular reason why 
>> > hwmon_attr_show_string passes only an "empty" pointer(pointer) to
>the ops-
>> > >read_string function rather than the buffer itself?
>> > 
>> > Wouldn't this mean that in ops->read_string I'd have to reserve
>some space for 
>> > the value on the heap (and taking care to free it somewhere, since
>returning 
>> > an address on the stack is bad idea), instead of calling
>sprintf(buf, "%s\n", 
>> > s) directly?
>> > 
>> > With the current implementation I have to sprintf it into my local
>buffer and 
>> > you sprintf it again into the final buffer.
>>
>> The idea was that the called code would return a pointer to a
>constant string,
>> ie one that isn't changing from call to call.
>
>In that case, what about the following change?
>
>Subject: hwmon: Constify str parameter of hwmon_ops->read_string
>
>The read_string callback is supposed to retrieve a pointer to a
>constant string.

I would think that clarifies the situation and also gets rid warnings about dropping const qualifier at the point of assignment.


Reviewed-by: Peter Huewe <peterhuewe@....de>


>
>Signed-off-by: Jean Delvare <jdelvare@...e.de>
>---
> drivers/hwmon/hwmon.c |    2 +-
> include/linux/hwmon.h |    2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
>--- linux-4.10.orig/drivers/hwmon/hwmon.c	2017-02-19 23:34:00.000000000
>+0100
>+++ linux-4.10/drivers/hwmon/hwmon.c	2017-03-07 08:22:27.784527968
>+0100
>@@ -186,7 +186,7 @@ static ssize_t hwmon_attr_show_string(st
> 				      char *buf)
> {
> 	struct hwmon_device_attribute *hattr = to_hwmon_attr(devattr);
>-	char *s;
>+	const char *s;
> 	int ret;
> 
> 	ret = hattr->ops->read_string(dev, hattr->type, hattr->attr,
>--- linux-4.10.orig/include/linux/hwmon.h	2017-02-19 23:34:00.000000000
>+0100
>+++ linux-4.10/include/linux/hwmon.h	2017-03-07 08:21:28.247998585
>+0100
>@@ -336,7 +336,7 @@ struct hwmon_ops {
> 	int (*read)(struct device *dev, enum hwmon_sensor_types type,
> 		    u32 attr, int channel, long *val);
> 	int (*read_string)(struct device *dev, enum hwmon_sensor_types type,
>-		    u32 attr, int channel, char **str);
>+		    u32 attr, int channel, const char **str);
> 	int (*write)(struct device *dev, enum hwmon_sensor_types type,
> 		     u32 attr, int channel, long val);
> };

-- 
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