linux-kernel - Re: [PATCH 03/11] drm/amdgpu: convert amdgpu_vm

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Date:   Fri, 4 Oct 2019 14:39:40 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Michel Lespinasse <walken@...gle.com>,
        "Koenig, Christian" <Christian.Koenig@....com>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Davidlohr Bueso <dbueso@...e.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Jerome Glisse <jglisse@...hat.com>,
        "amd-gfx@...ts.freedesktop.org" <amd-gfx@...ts.freedesktop.org>,
        Daniel Vetter <daniel@...ll.ch>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH 03/11] drm/amdgpu: convert amdgpu_vm_it to half closed
 intervals

Hi Michel,

Am 04.10.19 um 13:36 schrieb Michel Lespinasse:
> On Fri, Oct 04, 2019 at 06:54:54AM +0000, Koenig, Christian wrote:
>> Am 03.10.19 um 22:18 schrieb Davidlohr Bueso:
>>> The amdgpu_vm interval tree really wants [a, b) intervals,
>> NAK, we explicitly do need an [a, b[ interval here.
> Hi Christian,
>
> Just wanted to confirm where you stand on this patch, since I think
> you reconsidered your initial position after first looking at 9/11
> from this series.
>
> I do not know the amdgpu code well, but I think the changes should be
> fine - in struct amdgpu_bo_va_mapping, the "end" field will hold what
> was previously stored in the "last" field, plus one. The expectation
> is that overflows should not be an issue there, as "end" is explicitly
> declared as an uint64, and as the code was previously computing
> "last + 1" in many places.
>
> Does that seem workable to you ?

No, we computed last + 1 in a couple of debug places were it doesn't 
hurt us and IIRC we currently cheat a bit because we use pfn instead of 
addresses on some other places.

But that is only a leftover from radeon and we need to fix that sooner 
or later, cause essentially the physical address space of the device is 
really full 64bits, e.g. 0x0-0xffffffffffffffff.

So that only fits into a 64bit int when we use half open/closed 
intervals, but would wrap around to zero if we use a closed interval.

I initially thought that the set was changing the interval tree into 
always using a closed interval, but that seems to have been a 
misunderstanding.

Regards,
Christian.

>
> Thanks,
>