Re: [PATCH 6/8] drm: Add a drm_get_unmapped_area() helper

[Christian KÃnig]

Am 04.12.19 um 13:32 schrieb Thomas HellstrÃm (VMware):
> On 12/4/19 1:08 PM, Christian KÃnig wrote:
> > Am 04.12.19 um 12:36 schrieb Thomas HellstrÃm (VMware):
> > > On 12/4/19 12:11 PM, Christian KÃnig wrote:
> > > > Am 03.12.19 um 14:22 schrieb Thomas HellstrÃm (VMware):
> > > > > From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> > > > >
> > > > > This helper is used to align user-space buffer object addresses to
> > > > > huge page boundaries, minimizing the chance of alignment mismatch
> > > > > between user-space addresses and physical addresses.
> > > >
> > > > Mhm, I'm wondering if that is really such a good idea.
> > >
> > > Could you elaborate? What drawbacks do you see?
> >
> > Main problem for me seems to be that I don't fully understand what 
> > the get_unmapped_area callback is doing.
>
> It makes sure that, if there is a chance that we could use huge 
> page-table entries, virtual address huge page boundaries are perfectly 
> aligned to physical address huge page boundaries, which is if not a 
> CPU hardware requirement, at least a kernel requirement currently.
>
>
> > For example why do we need to use drm_vma_offset_lookup_locked() to 
> > adjust the pgoff?
> >
> > The mapped offset should be completely irrelevant for finding some 
> > piece of userspace address space or am I totally off here?
>
>
> Because the unmodified pgoff assumes that physical address boundaries 
> are perfectly aligned with file offset boundaries, which is typical 
> for all other subsystems.
>
> That's not true for TTM, however, where a buffer object start physical 
> address may be huge page aligned, but the file offset is always page 
> aligned. We could of course change that to align also file offsets to 
> huge page size boundaries, but with the above adjustment, that's not 
> needed. I opted for the adjustment.

I would opt for aligning the file offsets instead.

Now that you explained it that the rest of the kernel enforces this 
actually makes sense.

Regards,
Christian.

> Thanks,
>
> Thomas