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

[Matthew Brost <matthew.brost@intel.com>]

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.

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