[Intel-gfx] [PATCH v4 2/4] drm/i915: Use the vma resource as argument for gtt binding / unbinding

[Zeng, Oak]

Regards,
Oak

> -----Original Message-----
> From: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Sent: January 4, 2022 3:29 AM
> To: Zeng, Oak <oak.zeng at intel.com>; intel-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org
> Cc: Auld, Matthew <matthew.auld at intel.com>
> Subject: Re: [Intel-gfx] [PATCH v4 2/4] drm/i915: Use the vma resource as argument for gtt binding / unbinding
> 
> Hi, Oak.
> 
> On 1/4/22 00:08, Zeng, Oak wrote:
> >
> > Regards,
> > Oak
> 
> Looks like your emails always start with "Regards, Oak". a misconfiguration?

My mail client (outlook) is set to automatically add a regards, when I compose new mail or reply email. Not a big problem 😊

> 
> 
> >> -----Original Message-----
> >> From: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> >> Sent: January 3, 2022 1:58 PM
> >> To: Zeng, Oak <oak.zeng at intel.com>; intel-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org
> >> Cc: Auld, Matthew <matthew.auld at intel.com>
> >> Subject: Re: [Intel-gfx] [PATCH v4 2/4] drm/i915: Use the vma resource as argument for gtt binding / unbinding
> >>
> >> Hi, Oak.
> >>
> >> On 1/3/22 19:17, Zeng, Oak wrote:
> >>> Regards,
> >>> Oak
> >>>
> >>>> -----Original Message-----
> >>>> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Thomas Hellström
> >>>> Sent: January 3, 2022 7:00 AM
> >>>> To: intel-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org
> >>>> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>; Auld, Matthew <matthew.auld at intel.com>
> >>>> Subject: [Intel-gfx] [PATCH v4 2/4] drm/i915: Use the vma resource as argument for gtt binding / unbinding
> >>>>
> >>>> When introducing asynchronous unbinding, the vma itself may no longer
> >>>> be alive when the actual binding or unbinding takes place.
> >>> Can we take an extra reference counter of the vma to keep the vma alive, until the actual binding/unbinding takes place?
> >> The point here is that that's not needed, and should be avoided.
> > Can you explain more why "keeping vma alive until unbinding takes place" should be avoided?
> >
> > As I understand it, your series introduce asynchronized unbinding. But since vma might be no longer alive at the time of unbinding.
> To overcome this difficulty, you introduce a vma resource structure and you guarantee vma resource is alive at bind/unbind time. So
> you can use vma resource for the bind/unbind operation. My question is, can we achieve the asynchronized unbinding still using vma
> structure by keeping vma structure alive ( by ref count it). This way the change should be much smaller (compared to this series). Why
> it is harmful to keep the vma alive? Maybe you have other reasons to introduce vma resource that I don't see.
> 
> When we allow asynchronous unbinding, it's allowed to immediately rebind
> the vma, possibly into the same gpu virtual address, but with different
> pages. And when doing that we don't want to block waiting for the unbind
> to execute. 

Imagine this sequence:

1. Virtual address a1 is bound to physical page p1
2. Unbind a1 from p1, asynchronous so actual unbind not happen yet
3. bind a1 to physical page p2, page table is changed, now a1 pointing to p2 in page table.
4. #2 now take place now - since in page table, a1 points to p2 now, does a1 point to scratch page after #4?

In fact, we could allow a large number of outstanding binds
> and unbinds for a vma, which makes the vma structure unsuitable to track
> this, since there will no longer be a single mapping between a set of
> active pages and a vma, or a virtual gpu range and a vma.

I agree that if pages - vma - virtual gpu range is not 1:1:1 mapping, we need introduce a finer-grained vma resource to for the non-1:1 mapping. I also understand the asynchronous unbinding utilize the virtual address space more effectively. But my feeling is that this non-1:1 mapping makes our program hard to understand and maintain. Since this non- 1:1 mapping is introduced by asynchronous binding/unbinding, maybe the real question here is, is it really benefit to introduce asynchronous unbinding?

I am still not familiar enough to the codes. I suggest other experts to take a look also. @Bloomfield, Jon @Vetter, Daniel @Wilson, Chris P.

Regards,
Oak
> 
> Thanks,
> 
> /Thomas
> 
> >
> > Regards,
> > Oak
> >
> >   If the
> >> vma is no longer alive, that means nobody uses it anymore, but the GPU
> >> may still have work in the pipe that references the GPU virtual address.
> >>
> >> /Thomas.
> >>