Re: Question about hwmon_attr_show_string

Guenter Roeck
2017-03-07T15:05:09Z

On 03/07/2017 01:08 AM, Jean Delvare wrote:
> 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@suse.de>

Makes sense. I'll add it to -next.

Thanks,
Guenter

> ---
> 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);
> };
>
>