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

[Ruben Wauters <rubenru09@aol.com>]

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On Fri, 2026-10-30 at 22:07 +0000, Ruben Wauters wrote:
> 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@vger.kernel.org
> > Co-developed-by: Bin Yu <byu@xidian.edu.cn>
> > Signed-off-by: Bin Yu <byu@xidian.edu.cn>
> > Co-developed-by: Cheng Wen <wencheng@xidian.edu.cn>
> > Signed-off-by: Cheng Wen <wencheng@xidian.edu.cn>
> > Signed-off-by: YuanZhe Liu <25031212351@stu.xidian.edu.cn>
> Acked-by: Ruben Wauters <rubenru09@aol.com>

Applied to drm-misc-fixes, thanks for the patch
> 
> 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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