[PATCH v2] drm: add a check to verify the size alignment

[Matthew Auld]

On Tue, 29 Mar 2022 at 12:17, Arunpravin Paneer Selvam
<arunpravin.paneerselvam at amd.com> wrote:
>
>
>
> On 23/03/22 1:15 pm, Christian König wrote:
> > Am 23.03.22 um 08:34 schrieb Arunpravin Paneer Selvam:
> >> Add a simple check to reject any size not aligned to the
> >> min_page_size.
> >>
> >> handle instances when size is not aligned with the min_page_size.
> >> Unigine Heaven has allocation requests for example required pages
> >> are 257 and alignment request is 256. To allocate the left over 1
> >> page, continues the iteration to find the order value which is 0
> >> and when it compares with min_order = 8, triggers the BUG_ON(order
> >> < min_order). To avoid this problem, we added a simple check to
> >> return -EINVAL if size is not aligned to the min_page_size.
> >>
> >> v2: Added more details to the commit description
> >>
> >> Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam at amd.com>
> >> Suggested-by: Matthew Auld <matthew.auld at intel.com>
> >> ---
> >>   drivers/gpu/drm/drm_buddy.c | 3 +++
> >>   1 file changed, 3 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
> >> index 72f52f293249..b503c88786b0 100644
> >> --- a/drivers/gpu/drm/drm_buddy.c
> >> +++ b/drivers/gpu/drm/drm_buddy.c
> >> @@ -661,6 +661,9 @@ int drm_buddy_alloc_blocks(struct drm_buddy *mm,
> >>      if (range_overflows(start, size, mm->size))
> >>              return -EINVAL;
> >>
> >> +    if (WARN_ON(!IS_ALIGNED(size, min_page_size)))
> >> +            return -EINVAL;
> >> +
> >
> > I'm not that happy with the handling here.
> >
> > See a minimum page size larger than the requested size is perfectly
> > valid, it just means that the remaining pages needs to be trimmed.
> >
> > In other words when the request is to allocate 1 page with an alignment
> > of 256 we just need to give the remaining 255 pages back to the allocator.
>
> In one of the previous mail Matthew explained that i915 expects to
> return -EINVAL error code if size is not aligned to min_page_size.

We could also move the WARN_ON() into i915 as a separate patch, and
just change the default buddy behaviour to transparently handle the
rounding + trim, if you prefer. I don't have a strong opinion.

>
> can we just modify in amdgpu code where we round_up the size to the
> min_page_size value and keep this error handling in drm_buddy.c?
> >
> > Regards,
> > Christian.
> >
> >>      /* Actual range allocation */
> >>      if (start + size == end)
> >>              return __drm_buddy_alloc_range(mm, start, size, blocks);
> >>
> >> base-commit: 056d47eaf6ea753fa2e21da31f9cbd8b721bbb7b
> >