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

[Dave Gordon <david.s.gordon@intel.com>]

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).

> 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.

.Dave.
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