linux-kernel - Re: [PATCH v2] iio: st_lsm6dsx: Replace scnprintf with sysfs

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Message-ID: <22754870.EfDdHjke4D@mbox>
Date: Tue, 08 Jul 2025 12:20:04 +0530
From: akshay bansod <akbansd@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Andy Shevchenko <andriy.shevchenko@...el.com>,
 Lorenzo Bianconi <lorenzo@...nel.org>, David Lechner <dlechner@...libre.com>,
 Nuno Sá <nuno.sa@...log.com>,
 Andy Shevchenko <andy@...nel.org>,
 linux-kernel-mentees@...ts.linuxfoundation.org, skhan@...uxfoundation.org,
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iio: st_lsm6dsx: Replace scnprintf with sysfs_emit

On Sunday, 6 July 2025 10:00 am +0530 Jonathan Cameron wrote:
> On Thu, 03 Jul 2025 22:28:13 +0530
> akshay bansod <akbansd@...il.com> wrote:
> 
> > On Thursday, 3 July 2025 10:12 pm +0530 Andy Shevchenko wrote:
> > > On Thu, Jul 03, 2025 at 11:08:59AM +0530, Akshay Bansod wrote:  
> > > > Update the sysfs interface for sampling frequency and scale attributes.
> > > > Replace `scnprintf()` with `sysfs_emit_at()` which is PAGE_SIZE-aware
> > > > and recommended for use in sysfs.  
> > > 
> > > 'must' is stronger than 'recommendation'.
> > > Of has the documentation been changed lately?
> > > 
> > > ...
> > >   
> > > > st_lsm6dsx_sysfs_sampling_frequency_avail(struct device *dev,  
> > >   
> > > >  	odr_table = &sensor->hw->settings->odr_table[sensor->id];
> > > >  	for (i = 0; i < odr_table->odr_len; i++)
> > > > -		len += scnprintf(buf + len, PAGE_SIZE - len, "%d.%03d ",
> > > > -				 odr_table->odr_avl[i].milli_hz / 1000,
> > > > -				 odr_table->odr_avl[i].milli_hz % 1000);
> > > > +		len += sysfs_emit_at(buf, len, "%d.%03d ",
> > > > +				     odr_table->odr_avl[i].milli_hz / 1000,
> > > > +				     odr_table->odr_avl[i].milli_hz % 1000);
> > > >  	buf[len - 1] = '\n';  
> > > 
> > > My gosh, this is error prone. I'm wondering when some CIs will start to
> > > complain on this line. But this was already before your change...
> > >   
> > I'm planning to drop It entirely or should I replace it with another `sysfs_emit_at()` ?
> > I've seen other device driver returning space terminated buffers. Maybe I'm overlooking
> > something.
> 
> It is rather ugly currently but not a bug as such as we know we don't actually run
> out of space in the page (it would just overwrite last byte in that case so odd
> output, but not a bug) and that we always print something so just as you suggest
> sysfs_emit_at(buf, len - 1, "\n"); is safe.  It also checks under and overflow
> so that safe + hopefully won't trip up static analysis tools.
> 
understood. I'll revise the patch.

On a sidenode, I see a lot of repetitive code trying to write to a sysfs buffer
from a static array. for example

 drivers/iio/common/st_sensors/st_sensors_core.c:629
 drivers/iio/adc/vf610_adc.c:614
 drivers/iio/accel/adxl372.c:972

 ...

What if we export a symbol from industrialio-core.c which does something 
similar to

 drivers/iio/industrialio-core.c:815

 'iio_format_avail_list(char *buf, const int *vals,
				     int type, int length)'


but rather than taking integer array, it take `void* ptr` and `int stride` as
parameter. Then iterates from `vals` by `stride` for `count` times and typecast
the pointer and 'sysfs_emit` it.

static ssize_t iio_format_avail_list(char *buf, void *vals, 
			      int stride, int type, int count) {

	// iterate (void*) vals by stride and perform `sysfs_emit`
	
	void* ref = vals;
	for(int i = 0; i < count; i++){
	
		ref += stride;
		
		// typecast and write to buf using sysfs_emit
		...

	}
};


Thus, drivers can use this as follows.

--- a/drivers/iio/common/st_sensors/st_sensors_core.c
+++ b/drivers/iio/common/st_sensors/st_sensors_core.c
@@ -618,20 +618,11 @@ EXPORT_SYMBOL_NS(st_sensors_verify_id, "IIO_ST_SENSORS");
 ssize_t st_sensors_sysfs_sampling_frequency_avail(struct device *dev,
                                struct device_attribute *attr, char *buf)
 {
-       int i, len = 0;
        struct iio_dev *indio_dev = dev_to_iio_dev(dev);
        struct st_sensor_data *sdata = iio_priv(indio_dev);
 
-       for (i = 0; i < ST_SENSORS_ODR_LIST_MAX; i++) {
-               if (sdata->sensor_settings->odr.odr_avl[i].hz == 0)
-                       break;
-
-               len += scnprintf(buf + len, PAGE_SIZE - len, "%d ",
-                               sdata->sensor_settings->odr.odr_avl[i].hz);
-       }
-       buf[len - 1] = '\n';
-
-       return len;
+       return iio_format_avail_list(buf, &sdata->sensor_settings->odr.odr_avl[0].hz,
+               sizeof(st_sensor_odr_avl), IIO_VAL_INT, ST_SENSORS_ODR_LIST_MAX);
 }

The details about the various types to cover is still unclear. 
But does this sounds feasible ? 

> > 
> > > >  	return len;  
> > > 
> > > ...
> > >   

...

> > 
> > 
> > 
> > 
> 

Regards, 
Akshay Bansod