Re: [PATCH] drm/gud: avoid divide error in gud_stats_debugfs

[Ruben Wauters]

On Thu, 2026-10-29 at 15:33 +0800, YuanZhe Liu wrote:
> gud_stats_debugfs() computes the compression ratio by dividing the
> accumulated uncompressed length by stats_actual_length. That counter
> is
> zero until the first framebuffer flush has completed, so reading the
> stats debugfs file on a display which negotiates LZ4 compression
> raises
> a divide error before any frame has been sent. The wrap-around reset
> in
> gud_flush_rect() also clears the counter for a moment, which a
> concurrent read can observe.
> 
> Copy the counter into a local variable and skip the ratio computation
> when it is zero.
> 
> Fixes: 40e1a70b4aed ("drm: Add GUD USB Display driver")
> Cc: stable@xxxxxxxxxxxxxxx
> Co-developed-by: Bin Yu <byu@xxxxxxxxxxxxx>
> Signed-off-by: Bin Yu <byu@xxxxxxxxxxxxx>
> Co-developed-by: Cheng Wen <wencheng@xxxxxxxxxxxxx>
> Signed-off-by: Cheng Wen <wencheng@xxxxxxxxxxxxx>
> Signed-off-by: YuanZhe Liu <25031212351@xxxxxxxxxxxxxxxxx>
Acked-by: Ruben Wauters <rubenru09@xxxxxxx>

Sashiko highlighted a potential issue with reading stats_actual_length
twice when a flush_rect occurs between them. This *could* still trigger
a divide error, however, this implementation is still better than
current. The best thing to do would be to put a READ_ONCE when
stats_actual_length is read.

This patch is better than the current implementation, and I don't think
many compilers would optimise it to two memory reads? But it is
possible, and compiler optimisation is a fairly complicated topic, so
if you do wish you can add the READ_ONCE, but I'm also happy for it to
go in without, as it is better than current.

Ruben
> ---
>  drivers/gpu/drm/gud/gud_drv.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/gud/gud_drv.c
> b/drivers/gpu/drm/gud/gud_drv.c
> index 3a1b9e2a2eaa..85ddb302d2ba 100644
> --- a/drivers/gpu/drm/gud/gud_drv.c
> +++ b/drivers/gpu/drm/gud/gud_drv.c
> @@ -329,10 +329,17 @@ static int gud_stats_debugfs(struct seq_file
> *m, void *data)
>  	seq_puts(m, "\n");
>  
>  	if (gdrm->compression) {
> +		u64 actual_length = gdrm->stats_actual_length;
>  		u64 remainder;
> -		u64 ratio = div64_u64_rem(gdrm->stats_length, gdrm-
> >stats_actual_length,
> -					  &remainder);
> -		u64 ratio_frac = div64_u64(remainder * 10, gdrm-
> >stats_actual_length);
> +		u64 ratio, ratio_frac;
> +
> +		if (!actual_length) {
> +			seq_puts(m, "Compression ratio: n/a\n");
> +			return 0;
> +		}
> +
> +		ratio = div64_u64_rem(gdrm->stats_length,
> actual_length, &remainder);
> +		ratio_frac = div64_u64(remainder * 10,
> actual_length);
>  
>  		seq_printf(m, "Compression ratio: %llu.%llu\n",
> ratio, ratio_frac);
>  	}

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