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

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

On Tue, Nov 26, 2024 at 09:19:47AM +0100, Christian König wrote:
> 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.
>

Thanks, reposted with code inside ttm_bo_vm.c. Any objection to merging
entire series through drm-xe-next and then backporting single TTM patch
drm-misc-next?

Matt

> 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
>