Re: Feature Request: Ability to decode bus/dma address back into physical address

[Jerome Glisse <j.glisse-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Wed, Aug 02, 2017 at 07:23:58PM +0200, Christian König wrote:
> Am 02.08.2017 um 19:13 schrieb Jerome Glisse:
> > On Wed, Aug 02, 2017 at 07:05:11PM +0200, Christian König wrote:
> > > Am 02.08.2017 um 18:43 schrieb Jerome Glisse:
> > > > On Wed, Aug 02, 2017 at 10:26:40AM +0200, Christian König wrote:
> > > > > [SNIP]
> > > > So to summarize you are saying you do not trust the value you get from
> > > > pci_map_page() ?
> > > Well, what we don't trust is that we actually get this value correctly into
> > > our page tables.
> > > 
> > > > If not then i stress again that you have all the informations you need
> > > > inside the amdgpu driver. You can take the same scheme i propose to
> > > > dump ttm.dma_address[] and compare against content of GPU page table.
> > > Yes, exactly. But then again we have the mapping page to dma-address
> > > (because that is what drivers usually need), but what we need for debugging
> > > is a map with the info dma-address to page.
> > Why would you need the reverse ? You have a GPU virtual address do the following:
> > GPU vaddr -> GPU page table entrie -> bus address
> > GPU vaddr -> bo_va_mapping -> bo_va -> bo -> page -> dma/bus address
> 
> First of all the VM housekeeping structures keep the state as it is supposed
> to be on the next command submission and so the top of the pipeline, but the
> state of the page tables represent to bottom of the pipeline.
> 
> Second you can't access the BO from it's virtual address, the BO mapping is
> protected by the BO reservation lock. So when I want to lockup the BO I
> would need to lock the BO first - chicken and egg problem. That is of course
> solvable, but not something I would really like to do for a debugging
> feature.

Tom said that the GPU is stop and thus there is nothing happening to the vm
nor any of the bo right ? So there is no need to protect anything. If you
still allow thing to change vm (map/unmap bo ...) how do you expect to debug ?

Jérôme