Re: [PATCH] staging: fbtft: Use sysfs_emit_at() to print to sysfs file

[David Laight]

On Wed, 3 Jun 2026 10:49:11 +0300
Andy Shevchenko <andriy.shevchenko@xxxxxxxxx> wrote:

> On Wed, Jun 03, 2026 at 10:34:21AM +0300, Dan Carpenter wrote:
> > This scnprintf() uses the wrong limit.  It should be "PAGE_SIZE - len"
> > instead of just PAGE_SIZE.  We're not going to hit the limit in real
> > life since we are printing at most FBTFT_GAMMA_MAX_VALUES_TOTAL (128)
> > u32 values, however, it's still worth fixing.
> > 
> > Use sysfs_emit_at() to fix this since this is a sysfs file.  
> 
> OK,
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
> 
> ...
> 
> >  	for (i = 0; i < par->gamma.num_curves; i++) {
> >  		for (j = 0; j < par->gamma.num_values; j++)
> > -			len += scnprintf(&buf[len], PAGE_SIZE,
> > +			len += sysfs_emit_at(buf, len,
> >  			     "%04x ", curves[i * par->gamma.num_values + j]);  
> 
> Can we switch to use hex_dump_to_buffer() at some point?
> 

That gets hard when you really want to aim for something nearer seq_printf().

Oh, the way the loops in the code are written doesn't look like
it gives the compiler much chance of optimising it very well.
This is probably equivalent:
	for (i = par->gamma.num_curves; i--;) {
		for (j = par->gamma.num_values; j--;)
			len += sysfs_emit_at(buf, len, "%04x ", *curves++);
		buf[len - 1] = '\n';
	}
and will generate much better code.
(Although most of the cost inside the snprintf() function - which is horrid.)
num_values better be non-zero.

-- David