Re: [PATCH 01/15] drm/i915: Add i915_gem_object_write() to i915_gem.c

Chris Wilson
2015-06-22T12:37:43Z

On Mon, Jun 22, 2015 at 12:59:00PM +0100, Dave Gordon wrote:
> On 19/06/15 09:44, Chris Wilson wrote:
> > On Thu, Jun 18, 2015 at 07:07:46PM +0100, Dave Gordon wrote:
> >> On 18/06/15 13:10, Chris Wilson wrote:
> >>> On Thu, Jun 18, 2015 at 12:49:55PM +0100, Dave Gordon wrote:
> >>>> On 17/06/15 13:02, Daniel Vetter wrote:
> >>>>> Domain handling is required for all gem objects, and the resulting bugs if
> >>>>> you don't for one-off objects are absolutely no fun to track down.
> >>>>
> >>>> Is it not the case that the new object returned by
> >>>> i915_gem_alloc_object() is
> >>>> (a) of a type that can be mapped into the GTT, and
> >>>> (b) initially in the CPU domain for both reading and writing?
> >>>>
> >>>> So AFAICS the allocate-and-fill function I'm describing (to appear in
> >>>> next patch series respin) doesn't need any further domain handling.
> >>>
> >>> A i915_gem_object_create_from_data() is a reasonable addition, and I
> >>> suspect it will make the code a bit more succinct.
> >>
> >> I shall adopt this name for it :)
> >>
> >>> Whilst your statement is true today, calling set_domain is then a no-op,
> >>> and helps document how you use the object and so reduces the likelihood
> >>> of us introducing bugs in the future.
> >>> -Chris
> >>
> >> So here's the new function ... where should the set-to-cpu-domain go?
> >> After the pin_pages and before the sg_copy_from_buffer?
> >
> > Either, since the domain will not change whilst you have the lock,
> > but if you do it before get_pages() you will have a slightly easier
> > error path.
>
> OK, call to i915_gem_object_set_to_cpu_domain(obj, true) added right
> after the i915_gem_alloc_object(); also, since we now have multiple
> failure paths where the ability to distinguish them might be useful (and
> since this function is a public addition to the gem_object repertoire),
> I've made it return object-or-error-code, with the incomplete-copy case
> returning ERR_PTR(-EIO).

I'd stick to only using EIO when we have a GPU failure. Incomplete copy
is EFAULT.

> > Part of the reason why I want a function like this is so that I can
> > replace it with a stolen object and so need to write the data through a
> > temporary GGTT mapping. Speak now if you need more flags to the function
> > to prevent certain classes of objects being created.
> > -Chris
>
> For the GuC, the firmware image is written once by the CPU and
> thereafter read only by the DMA engine via a GGTT address; other uC
> devices might have different requirements e.g. the CSR/DMC doesn't have
> a DMA engine AFAIK and the f/w is transferred to the h/w via MMIO writes
> by the CPU. The primary reason for storing the image in a GEM object
> (rather than kmalloc'd space, as the DMC loader does) is to make it
> pageable; it's needed multiple times, as we have to reload the f/w after
> reset or on exit from low-power states, but not used the rest of the
> time. So the existing objects seem a good match.
>
> The GuC's pool and log objects, OTOH, must be permanently resident in
> RAM and permanently mapped via the GGTT above GUC_WOPCM_OFFSET. So for
> these it would be useful to have an allocator that *didn't* make the
> object shmfs-backed.

http://cgit.freedesktop.org/~ickle/linux-2.6/commit/?h=nightly&id=3f74a251aa9a3e5c1215226f7857ed53693c563f

Though I want to use stolen as much as is practically possible, however
without direct CPU access, stolen is very much an idle fancy.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre