Re: Question about hwmon_attr_show_string

Peter Huewe
2017-03-07T09:25:03Z

Am 7. März 2017 10:08:46 MEZ schrieb Jean Delvare <jdelvare@suse.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@gmx.de>

>
>Signed-off-by: Jean Delvare <jdelvare@suse.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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