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Subject: Re: nouveau broken on Riva TNT2 in 5.13.0-rc4: NULL pointer
 dereference in nouveau_bo_sync_for_device
To:     Ondrej Zary <linux@zary.sk>
Cc:     Ben Skeggs <bskeggs@redhat.com>, dri-devel@lists.freedesktop.org,
        nouveau@lists.freedesktop.org, linux-kernel@vger.kernel.org
References: <202106052143.52488.linux@zary.sk>
 <202106090857.42133.linux@zary.sk>
 <1c4a7360-57e3-c75a-c729-1432db5b90b9@amd.com>
 <202106090910.51188.linux@zary.sk>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
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Date:   Wed, 9 Jun 2021 11:21:05 +0200
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Am 09.06.21 um 09:10 schrieb Ondrej Zary:
> On Wednesday 09 June 2021, Christian König wrote:
>> Am 09.06.21 um 08:57 schrieb Ondrej Zary:
>>> [SNIP]
>>>> Thanks for the heads up. So the problem with my patch is already fixed,
>>>> isn't it?
>>> The NULL pointer dereference in nouveau_bo_wr16 introduced in
>>> 141b15e59175aa174ca1f7596188bd15a7ca17ba was fixed by
>>> aea656b0d05ec5b8ed5beb2f94c4dd42ea834e9d.
>>>
>>> That's the bug I hit when bisecting the original problem:
>>> NULL pointer dereference in nouveau_bo_sync_for_device
>>> It's caused by:
>>> # first bad commit: [e34b8feeaa4b65725b25f49c9b08a0f8707e8e86] drm/ttm: merge ttm_dma_tt back into ttm_tt
>> Good that I've asked :)
>>
>> Ok that's a bit strange. e34b8feeaa4b65725b25f49c9b08a0f8707e8e86 was
>> created mostly automated.
>>
>> Do you have the original backtrace of that NULL pointer deref once more?
> The original backtrace is here: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.org%2Flkml%2F2021%2F6%2F5%2F350&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7Ce905b6bd2aa842ace15508d92b15b96d%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637588195000729460%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=zFqheBbJcOHtYgqG%2Fs63AT1dwuk4REmUDJWHvzaLAlc%3D&amp;reserved=0

And the problem is that ttm_dma->dma_address is NULL, right? Mhm, I 
don't see how that can happen since nouveau is using ttm_sg_tt_init().

Apart from that what nouveau does here is rather questionable since you 
need a coherent architecture for most things anyway, but that's not what 
we are trying to fix here.

Can you try to narrow down if ttm_sg_tt_init is called before calling 
this function for the tt object in question?

Thanks,
Christian.