[PATCH] drm/amd/display: Fix division by zero in setup_dsc_config

[PATCH] drm/amd/display: Fix division by zero in setup_dsc_config

[Jose Fernandez]

When slice_height is 0, the division by slice_height in the calculation
of the number of slices will cause a division by zero driver crash. This
leaves the kernel in a state that requires a reboot. This patch adds a
check to avoid the division by zero.

The stack trace below is for the 6.8.4 Kernel. I reproduced the issue on
a Z16 Gen 2 Lenovo Thinkpad with a Apple Studio Display monitor
connected via Thunderbolt. The amdgpu driver crashed with this exception
when I rebooted the system with the monitor connected.

kernel: ? die (arch/x86/kernel/dumpstack.c:421 arch/x86/kernel/dumpstack.c:434 arch/x86/kernel/dumpstack.c:447)
kernel: ? do_trap (arch/x86/kernel/traps.c:113 arch/x86/kernel/traps.c:154)
kernel: ? setup_dsc_config (drivers/gpu/drm/amd/amdgpu/../display/dc/dsc/dc_dsc.c:1053) amdgpu
kernel: ? do_error_trap (./arch/x86/include/asm/traps.h:58 arch/x86/kernel/traps.c:175)
kernel: ? setup_dsc_config (drivers/gpu/drm/amd/amdgpu/../display/dc/dsc/dc_dsc.c:1053) amdgpu
kernel: ? exc_divide_error (arch/x86/kernel/traps.c:194 (discriminator 2))
kernel: ? setup_dsc_config (drivers/gpu/drm/amd/amdgpu/../display/dc/dsc/dc_dsc.c:1053) amdgpu
kernel: ? asm_exc_divide_error (./arch/x86/include/asm/idtentry.h:548)
kernel: ? setup_dsc_config (drivers/gpu/drm/amd/amdgpu/../display/dc/dsc/dc_dsc.c:1053) amdgpu
kernel: dc_dsc_compute_config (drivers/gpu/drm/amd/amdgpu/../display/dc/dsc/dc_dsc.c:1109) amdgpu

After applying this patch, the driver no longer crashes when the monitor
is connected and the system is rebooted. I believe this is the same
issue reported for 3113.

Signed-off-by: Jose Fernandez <josef@netflix.com>
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3113
---
 drivers/gpu/drm/amd/display/dc/dsc/dc_dsc.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/dsc/dc_dsc.c b/drivers/gpu/drm/amd/display/dc/dsc/dc_dsc.c
index ac41f9c0a283..597d5425d6cb 100644
--- a/drivers/gpu/drm/amd/display/dc/dsc/dc_dsc.c
+++ b/drivers/gpu/drm/amd/display/dc/dsc/dc_dsc.c
@@ -1055,7 +1055,12 @@ static bool setup_dsc_config(
 	if (!is_dsc_possible)
 		goto done;
 
-	dsc_cfg->num_slices_v = pic_height/slice_height;
+	if (slice_height > 0)
+		dsc_cfg->num_slices_v = pic_height/slice_height;
+	else {
+		is_dsc_possible = false;
+		goto done;
+	}
 
 	if (target_bandwidth_kbps > 0) {
 		is_dsc_possible = decide_dsc_target_bpp_x16(
-- 
2.44.0

Re: [PATCH] drm/amd/display: Fix division by zero in setup_dsc_config

[Markus Elfring]

…
> +++ b/drivers/gpu/drm/amd/display/dc/dsc/dc_dsc.c
> @@ -1055,7 +1055,12 @@ static bool setup_dsc_config(
>  	if (!is_dsc_possible)
>  		goto done;
>
> -	dsc_cfg->num_slices_v = pic_height/slice_height;
> +	if (slice_height > 0)
> +		dsc_cfg->num_slices_v = pic_height/slice_height;
> +	else {
> +		is_dsc_possible = false;
> +		goto done;
> +	}
>
>  	if (target_bandwidth_kbps > 0) {
>  		is_dsc_possible = decide_dsc_target_bpp_x16(

I suggest to take another coding style concern into account.
Please use curly brackets for both if branches.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/coding-style.rst?h=v6.9-rc5#n213

Regards,
Markus