re: drm/amd/display: make PSR static screen entry within 30 ms

re: drm/amd/display: make PSR static screen entry within 30 ms

[Colin Ian King]

Hi,

Static analysis with Coverity has detected a division by zero in the
following commit:

commit 5b5abe9526073ccbf3032d27b5864520829cdd9c
Author: Anthony Koo <Anthony.Koo@amd.com>
Date:   Mon Dec 9 17:26:34 2019 -0500

    drm/amd/display: make PSR static screen entry within 30 ms

Specifically:

       unsigned int vsync_rate_hz = 0;
       struct dc_static_screen_params params = {0};
       /* Calculate number of static frames before generating interrupt to
        * enter PSR.
        */
       unsigned int frame_time_microsec = 1000000 / vsync_rate_hz;

vsync_rate_hz is zero, and frame_time_microsec is being assigned a value
that is being divided by zero.  I'm not sure why this is coded this way
and not sure what the fix is, hence I'm reporting the issue.

Colin
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[PATCH] drm/amd/display: Fix psr static frames calculation

[Harry Wentland]

From: Roman Li <Roman.Li@amd.com>

[Why]
Driver crash with psr feature enabled due to divide-by-zero error.
This is a regression after rework to calculate static screen frame
number entry time.

[How]
Correct order of operations to avoid divide-by-zero.

Cc: Colin Ian King <colin.king@canonical.com>
Fixes: 5b5abe952607 drm/amd/display: make PSR static screen entry within 30 ms
Cc: stable@vger.kernel.org
Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index eed3ed7180fd..61c36c1520c2 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -8491,7 +8491,6 @@ bool amdgpu_dm_psr_enable(struct dc_stream_state *stream)
 	/* Calculate number of static frames before generating interrupt to
 	 * enter PSR.
 	 */
-	unsigned int frame_time_microsec = 1000000 / vsync_rate_hz;
 	// Init fail safe of 2 frames static
 	unsigned int num_frames_static = 2;
 
@@ -8506,8 +8505,10 @@ bool amdgpu_dm_psr_enable(struct dc_stream_state *stream)
 	 * Calculate number of frames such that at least 30 ms of time has
 	 * passed.
 	 */
-	if (vsync_rate_hz != 0)
+	if (vsync_rate_hz != 0) {
+		unsigned int frame_time_microsec = 1000000 / vsync_rate_hz;
 		num_frames_static = (30000 / frame_time_microsec) + 1;
+	}
 
 	params.triggers.cursor_update = true;
 	params.triggers.overlay_update = true;
-- 
2.25.0

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Re: [PATCH] drm/amd/display: Fix psr static frames calculation

[Alex Deucher]

On Fri, Jan 24, 2020 at 2:23 PM Harry Wentland <harry.wentland@amd.com> wrote:
>
> From: Roman Li <Roman.Li@amd.com>
>
> [Why]
> Driver crash with psr feature enabled due to divide-by-zero error.
> This is a regression after rework to calculate static screen frame
> number entry time.
>
> [How]
> Correct order of operations to avoid divide-by-zero.
>
> Cc: Colin Ian King <colin.king@canonical.com>
> Fixes: 5b5abe952607 drm/amd/display: make PSR static screen entry within 30 ms
> Cc: stable@vger.kernel.org
> Signed-off-by: Roman Li <roman.li@amd.com>
> Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>

Acked-by: Alex Deucher <alexander.deucher@amd.com>

> ---
>  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> index eed3ed7180fd..61c36c1520c2 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -8491,7 +8491,6 @@ bool amdgpu_dm_psr_enable(struct dc_stream_state *stream)
>         /* Calculate number of static frames before generating interrupt to
>          * enter PSR.
>          */
> -       unsigned int frame_time_microsec = 1000000 / vsync_rate_hz;
>         // Init fail safe of 2 frames static
>         unsigned int num_frames_static = 2;
>
> @@ -8506,8 +8505,10 @@ bool amdgpu_dm_psr_enable(struct dc_stream_state *stream)
>          * Calculate number of frames such that at least 30 ms of time has
>          * passed.
>          */
> -       if (vsync_rate_hz != 0)
> +       if (vsync_rate_hz != 0) {
> +               unsigned int frame_time_microsec = 1000000 / vsync_rate_hz;
>                 num_frames_static = (30000 / frame_time_microsec) + 1;
> +       }
>
>         params.triggers.cursor_update = true;
>         params.triggers.overlay_update = true;
> --
> 2.25.0
>
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> amd-gfx@lists.freedesktop.org
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