[Intel-xe] [PATCH v3] drm/xe/bo: Return object bo size on create

[Zbigniew Kempczyński]

On Mon, Jun 26, 2023 at 06:16:06PM +0200, Thomas Hellström wrote:
> 
> On 6/26/23 17:22, Zbigniew Kempczyński wrote:
> > On Mon, Jun 26, 2023 at 04:55:29PM +0200, Thomas Hellström wrote:
> > > Hi, Zbigniew,
> > > 
> > > On 6/26/23 16:42, Zbigniew Kempczyński wrote:
> > > > Driver may alter bo size requested by the user. Return real object
> > > > size to make userspace aware how to arrange vm bindings.
> > > I think for Xe VM_BIND, the space needed for a bo with given size must be
> > > 
> > > ALIGN_TO(bo->size, xe_get_default_alignment())
> > > 
> > > (xe_get_default_alignment() being an igt xe lib function).
> > You mean I can't glue 4K system object with 64K one after another,
> > for example:
> > 
> > * bo on system: offset = 0x1000, size 0x1000
> > * bo on vram:   offset = 0x2000, size 0x10000
> > 
> > This won't work as vram object should be aligned on vm to 0x10000?
> 
> Yes that won't work, since a VRAM-capable bo needs 64K alignment.
> 
> Although the explanation above ignores that system-only bo objects appears
> to always allow 4K offset and size, which of course can be discussed.
> 
> So with the current uAPI, it appears the bind alignment and size could
> always be calculated using:
> 
> if (bo_has_VRAM_placement() && xe_get_default_alignment() > 4K) {
>    bind_size = ALIGN_TO(bo->size, xe_get_default_alignment());
>    offset_alignment = xe_get_default_alignment();
> } else {
>    bind_size = bo->size;
>    offset_alignment = 4K;
> }

offset_alignment = xe_get_default_alignment() is enough, as for
integrated we got only one region and min_page_size is 4K. For discrete
system objects just won't be adjacent (if I'm not wrong this lead to
bigger ppgtt).

> 
> What I'm a bit afraid of is that if we return a size different from
> bo->size, then UMD might start using that extra size for data, while it
> really shouldn't. Would it be possible to return an alignment instead that
> needs to be used by UMD for bind offset and bind size?

Technically we got those alignments (for regions) - they can be get from
min_page_size for some region. Problem I see is when user will create
an object with flags = SYSTEM | VRAM. How to properly do vmbind in case?
I mean sth like snippet below:

size = 0x2000;
bo = xe_bo_create_flags(fd, size, SYSTEM | VRAM);

If I don't know on which region this object is created how can I prepare
vm-bind range properly? May I use 0x2000 as size for object created
in vram or this gpu mapping should be aligned to 64K?

--
Zbigniew

> 
> /Thomas
> 
> > 
> > --
> > Zbigniew
> > 
> > > Even the padded bo->size returned with this patch is lower than the value
> > > above. If you use that value, will the value assigned below still be needed.
> > > 
> > > /Thomas
> > > 
> > > > Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski at intel.com>
> > > > Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
> > > > Reviewed-by: Matthew Brost <matthew.brost at intel.com>
> > > > ---
> > > >    drivers/gpu/drm/xe/xe_bo.c | 1 +
> > > >    1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> > > > index 71a14e1f25be..55dbb374a787 100644
> > > > --- a/drivers/gpu/drm/xe/xe_bo.c
> > > > +++ b/drivers/gpu/drm/xe/xe_bo.c
> > > > @@ -1744,6 +1744,7 @@ int xe_gem_create_ioctl(struct drm_device *dev, void *data,
> > > >    		return err;
> > > >    	args->handle = handle;
> > > > +	args->size = bo->size;
> > > >    	return 0;
> > > >    }