[PATCH v6 2/8] drm/ttm: Add ttm_bo_access

[Christian König]

Am 25.11.24 um 18:27 schrieb Matthew Brost:
> On Mon, Nov 25, 2024 at 05:19:54PM +0100, Christian König wrote:
>> Am 25.11.24 um 16:29 schrieb Matthew Brost:
>>> On Fri, Nov 15, 2024 at 10:27:59AM -0800, Matthew Brost wrote:
>>>> [SNIP]
>>>> We use this interface to read a BO marked with a dumpable flag during a
>>>> GPU hang in our error capture code. This is an internal KMD feature, not
>>>> directly exposed to user space. Would adding this helper be acceptable
>>>> for this use case? I can add kernel indicating the current restrictions
>>>> of the helper (do not directly expose to user space) too if that would
>>>> help.
>>>>
>>> Christian - ping on above.
>> Sorry, I will try to give those mailing list tasks a bit more time in before
>> the xmas holidays.
>>
>> That is an acceptable use case, but the problem is that this helper won't
>> work for that.
>>
>> See during a GPU hang you can't lock BOs, so how do you want to look into
>> their content with the peek helper?
>>
> Agree we cannot lock BO directly in GPU hang path (TDR). Our error
> capture code takes a snapshot of some the GPU state which is small and
> safe to capture in TDR and kicks a worker which opportunistically
> captures the VM state which has been marked to be captured. This is
> where the helper is called and it is safe to lock the BO.

Yeah that sounds like it should work.

No objections from my side for that use case, but I would rather like to 
keep the code inside ttm_bo_vm.c.

Crash dumping is usually something associated with the VMA even if it's 
a bit special here for the VM state.

Regards,
Christian.

>
> Matt
>
>> The only thing you could potentially do is to trylock the BO and then dump,
>> but that would most likely be a bit unreliable.
>>
>> Regards,
>> Christian.
>>
>>>> Matt