Re: Nouveau crashes in 4.6-rc on arm64

From: Alexandre Courbot
Date: Wed Apr 20 2016 - 00:35:24 EST

On 04/11/2016 04:22 PM, Alexandre Courbot wrote:
Hi Robin,

On 04/09/2016 03:46 AM, Robin Murphy wrote:
Hi Alex,

On 08/04/16 05:47, Alexandre Courbot wrote:
Hi Robin,

On 04/07/2016 08:50 PM, Robin Murphy wrote:

With 4.6-rc2 (and -rc1) I'm seeing Nouveau blowing up at boot, from the
look of it by dereferencing some offset from NULL inside
nouveau_fbcon_imageblit(). My setup is an old XFX 7600GT card plugged
into an ARM Juno r1 board, which works fine with 4.5 and earlier.

Attached are a couple of logs from booting arm64 defconfig plus DRM and
Nouveau enabled - the second also has framebuffer console rotation
turned on, which interestingly seems to move the point of failure, and
the display does eventually come up to show the tail end of the
panic in
that case.

I might be able to find time for a full bisection next week if isn't
something sufficiently obvious to anyone who knows this driver.

Looking at the log it is not clear to me what could be causing this. I
can boot 4.6-rc2 with a GM206 card without any issue. A bisect would
indeed be useful here.

OK, turns out the lure of writing something to remotely drive a Juno and
parse kernel bootlogs through an automatic bisection was too great to
resist on a Friday afternoon :D

Bisection came down to 1733a2ad3674("drm/nouveau/device/pci: set as
non-CPU-coherent on ARM64"), and sure enough reverting that removes the

Thanks for taking the time to bisect this. And apologies as it seems my
commit is the reason for your troubles.

The CPU coherency flag is used for two things: explicitly sync buffers
pages when required, and allocating buffers that are not explicitly
synced (like fences or pushbuffers) using the DMA API. For this latter
use, it also accesses the buffer's content using the mapping provided by
dma_alloc_coherent() instead of creating a new one. All nouveau_bos are
supposed to be written using nouveau_bo_rd32(), and this function
handles the case of an DMA-API allocated object by detecting that the
result of ttm_kmap_obj_virtual() is NULL.

But as it turns out, OUT_RINGp() also calls ttm_kmap_obj_virtual() in
order to perform a memcpy and uses its result directly - which means we
are doing memcpy on a NULL pointer. We never caught this because we
typically do not use Nouveau's fbcon with an ARM setup.

I don't really like this special access for coherent objects, and
actually had a patch in my tree to attempt to remove it (attached).
Although it is not the whole solution (see below), the issue should at
least not be visible with it applied - could you confirm?

Hi Robin, could you confirm whether the attached patch in my previous mail helps with your problem?