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

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

[Dan Carpenter]

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.

Fixes: c296d5f9957c ("staging: fbtft: core support")
Signed-off-by: Dan Carpenter <error27@gmail.com>
---
 drivers/staging/fbtft/fbtft-sysfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/fbtft/fbtft-sysfs.c b/drivers/staging/fbtft/fbtft-sysfs.c
index d05599d80011..343545e83a37 100644
--- a/drivers/staging/fbtft/fbtft-sysfs.c
+++ b/drivers/staging/fbtft/fbtft-sysfs.c
@@ -98,7 +98,7 @@ sprintf_gamma(struct fbtft_par *par, u32 *curves, char *buf)
 	mutex_lock(&par->gamma.lock);
 	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]);
 		buf[len - 1] = '\n';
 	}
-- 
2.53.0

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

[Andy Shevchenko]

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@intel.com>

...

>  	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?

-- 
With Best Regards,
Andy Shevchenko

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@intel.com> 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@intel.com>
> 
> ...
> 
> >  	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