On Wed, 17 Jan 2018 12:03:15 -0800 Eric Anholt eric@anholt.net wrote:

Boris Brezillon boris.brezillon@free-electrons.com writes:

...

When saving BOs in the hang state we skip one entry of the kernel_state->bo[] array, thus leaving it to NULL. This leads to a NULL pointer dereference when, later in this function, we iterate over all BOs to check their ->madv state.

Fixes: ca26d28bbaa3 ("drm/vc4: improve throughput by pipelining binning and rendering jobs") Cc: stable@vger.kernel.org Signed-off-by: Boris Brezillon boris.brezillon@free-electrons.com

---

drivers/gpu/drm/vc4/vc4_gem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vc4/vc4_gem.c b/drivers/gpu/drm/vc4/vc4_gem.c index 6c32c89a83a9..19ac7fe0e5db 100644 --- a/drivers/gpu/drm/vc4/vc4_gem.c +++ b/drivers/gpu/drm/vc4/vc4_gem.c @@ -208,7 +208,7 @@ vc4_save_hang_state(struct drm_device *dev) kernel_state->bo[j + prev_idx] = &bo->base.base; j++; }

• prev_idx = j + 1;
  

• prev_idx = j;  
  

Could we replace the whole "[j + prev_idx]" with a "[k++]" and maybe a WARN_ON_ONCE(k != state->bo_count) at the end?

Sure.

I really need to figure out if I can come up with a way to make IGT cases for GPU hangs on vc4, despite the validation.

I managed to trigger the NULL pointer dereference while debugging the perfmon stuff, but it's fixed now, so I don't have a way to easily force a reset.

I found a bug in GPU reset due to BCL hangs when doing vc5, but I don't have a testcase. Maybe some submit flags that overwrite the BCL or RCL to do an infinite loop?