linux-kernel - Re: Question about hwmon_attr_show

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Date:   Tue, 7 Mar 2017 10:08:46 +0100
From:   Jean Delvare <jdelvare@...e.de>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Peter Hüwe <PeterHuewe@....de>,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Question about hwmon_attr_show_string

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.

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);
 };


-- 
Jean Delvare
SUSE L3 Support