[PATCH 4/4] drm/i915/gem: Migrate to system at dma-buf map time

[Ruhl, Michael J]

>-----Original Message-----
>From: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>Sent: Friday, June 25, 2021 2:50 PM
>To: Ruhl, Michael J <michael.j.ruhl 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: [PATCH 4/4] drm/i915/gem: Migrate to system at dma-buf map
>time
>
>Hi, Mike,
>
>On 6/25/21 7:57 PM, Ruhl, Michael J wrote:
>>> -----Original Message-----
>>> From: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>>> Sent: Friday, June 25, 2021 1:52 PM
>>> To: Ruhl, Michael J <michael.j.ruhl 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: [PATCH 4/4] drm/i915/gem: Migrate to system at dma-buf
>map
>>> time
>>>
>>>
>>> On 6/25/21 7:38 PM, Ruhl, Michael J wrote:
>>>>> -----Original Message-----
>>>>> From: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>>>>> Sent: Friday, June 25, 2021 12:18 PM
>>>>> To: Ruhl, Michael J <michael.j.ruhl 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: [PATCH 4/4] drm/i915/gem: Migrate to system at dma-buf
>>> map
>>>>> time
>>>>>
>>>>> Hi, Michael,
>>>>>
>>>>> thanks for looking at this.
>>>>>
>>>>> On 6/25/21 6:02 PM, Ruhl, Michael J wrote:
>>>>>>> -----Original Message-----
>>>>>>> From: dri-devel <dri-devel-bounces at lists.freedesktop.org> On
>Behalf
>>> Of
>>>>>>> Thomas Hellström
>>>>>>> Sent: Thursday, June 24, 2021 2:31 PM
>>>>>>> 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: [PATCH 4/4] drm/i915/gem: Migrate to system at dma-buf
>map
>>>>> time
>>>>>>> Until we support p2p dma or as a complement to that, migrate data
>>>>>>> to system memory at dma-buf map time if possible.
>>>>>>>
>>>>>>> Signed-off-by: Thomas Hellström
><thomas.hellstrom at linux.intel.com>
>>>>>>> ---
>>>>>>> drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 9 ++++++++-
>>>>>>> 1 file changed, 8 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>>>>>>> b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>>>>>>> index 616c3a2f1baf..a52f885bc09a 100644
>>>>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>>>>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>>>>>>> @@ -25,7 +25,14 @@ static struct sg_table
>>>>> *i915_gem_map_dma_buf(struct
>>>>>>> dma_buf_attachment *attachme
>>>>>>> 	struct scatterlist *src, *dst;
>>>>>>> 	int ret, i;
>>>>>>>
>>>>>>> -	ret = i915_gem_object_pin_pages_unlocked(obj);
>>>>>>> +	ret = i915_gem_object_lock_interruptible(obj, NULL);
>>>>>> Hmm, I believe in most cases that the caller should be holding the
>>>>>> lock (object dma-resv) on this object already.
>>>>> Yes, I agree, In particular for other instances of our own driver,  at
>>>>> least since the dma_resv introduction.
>>>>>
>>>>> But I also think that's a pre-existing bug, since
>>>>> i915_gem_object_pin_pages_unlocked() will also take the lock.
>>>> Ouch yes.  Missed that.
>>>>
>>>>> I Think we need to initially make the exporter dynamic-capable to
>>>>> resolve this, and drop the locking here completely, as dma-buf docs says
>>>>> that we're then guaranteed to get called with the object lock held.
>>>>>
>>>>> I figure if we make the exporter dynamic, we need to migrate already at
>>>>> dma_buf_pin time so we don't pin the object in the wrong location.
>>>> The exporter as dynamic  (ops->pin is available) is optional, but importer
>>>> dynamic (ops->move_notify) is required.
>>>>
>>>> With that in mind, it would seem that there are three possible
>combinations
>>>> for the migrate to be attempted:
>>>>
>>>> 1) in the ops->pin function (export_dynamic != import_dynamic, during
>>> attach)
>>>> 2) in the ops->pin function (export_dynamic and
>>> !CONFIG_DMABUF_MOVE_NOTIFY) during mapping
>>>> 3) and possibly in ops->map_dma_buf (exort_dynamic iand
>>> CONFIG_DMABUF_MOVE_NOTIFY)
>>>> Since one possibility has to be in the mapping function, it seems that if we
>>>> can figure out the locking, that the migrate should probably be available
>>> here.
>>>> Mike
>>> So perhaps just to initially fix the bug, we could just implement NOP
>>> pin() and unpin() callbacks and drop the locking in map_attach() and
>>> replace it with an assert_object_held();
>> That is the sticky part of the move notify API.
>>
>> If you do the attach_dynamic you have to have an ops with move_notify.
>>
>> (https://elixir.bootlin.com/linux/v5.13-rc7/source/drivers/dma-buf/dma-
>buf.c#L730)
>>
>> If you don't have that, i.e. just the pin interface, the attach will be
>> rejected, and you will not get the callbacks.
>
>I understood that as the requirement for move_notify is only if the
>*importer* declares dynamic. A dynamic exporter could choose whether to
>call move_notify() on eviction or to pin and never evict. If the
>importer is non-dynamic, the core calls pin() and the only choice is to
>pin and never evict.
>
>So if we temporarily choose to pin and never evict for *everything*, (as
>the current code does now), I think we should be good for now, and then
>we can implement all fancy p2p and move_notify stuff on top of that.

/sigh.

You are correct.  I was mistakenly placing the pin API (dma_buf_ops) in the
attach_ops. 😐 Must be Friday.

Upon further reflection, I think that your path will work.

However, is doing a pin (with no locking) from the dma_buf_mapping any different
from using the pin API + export_dynamic?

M

>/Thomas
>
>
>>
>> So I think that the only thing we can do for now is to dop the locking and add
>the
>>
>> assert_object_held();
>>
>> M
>
>
>
>>
>>> /Thomas
>>>