Re: [Intel-gfx] [PATCH 00/15] HuC loading for DG2

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 13/06/2022 18:06, Ceraolo Spurio, Daniele wrote:
> On 6/13/2022 9:56 AM, Tvrtko Ursulin wrote:
>> On 13/06/2022 17:41, Ceraolo Spurio, Daniele wrote:
>>> On 6/13/2022 9:31 AM, Tvrtko Ursulin wrote:
>>>>
>>>> On 13/06/2022 16:39, Ceraolo Spurio, Daniele wrote:
>>>>> On 6/13/2022 1:16 AM, Tvrtko Ursulin wrote:
>>>>>>
>>>>>> On 10/06/2022 00:19, Daniele Ceraolo Spurio wrote:
>>>>>>> On DG2, HuC loading is performed by the GSC, via a PXP command. 
>>>>>>> The load
>>>>>>> operation itself is relatively simple (just send a message to the 
>>>>>>> GSC
>>>>>>> with the physical address of the HuC in LMEM), but there are timing
>>>>>>> changes that requires special attention. In particular, to send a 
>>>>>>> PXP
>>>>>>> command we need to first export the GSC driver and then wait for the
>>>>>>> mei-gsc and mei-pxp modules to start, which means that HuC load will
>>>>>>> complete after i915 load is complete. This means that there is a 
>>>>>>> small
>>>>>>> window of time after i915 is registered and before HuC is loaded
>>>>>>> during which userspace could submit and/or checking the HuC load 
>>>>>>> status,
>>>>>>> although this is quite unlikely to happen (HuC is usually loaded 
>>>>>>> before
>>>>>>> kernel init/resume completes).
>>>>>>> We've consulted with the media team in regards to how to handle 
>>>>>>> this and
>>>>>>> they've asked us to do the following:
>>>>>>>
>>>>>>> 1) Report HuC as loaded in the getparam IOCTL even if load is 
>>>>>>> still in
>>>>>>> progress. The media driver uses the IOCTL as a way to check if 
>>>>>>> HuC is
>>>>>>> enabled and then includes a secondary check in the batches to get 
>>>>>>> the
>>>>>>> actual status, so doing it this way allows userspace to keep working
>>>>>>> without changes.
>>>>>>>
>>>>>>> 2) Stall all userspace VCS submission until HuC is loaded. Stalls 
>>>>>>> are
>>>>>>> expected to be very rare (if any), due to the fact that HuC is 
>>>>>>> usually
>>>>>>> loaded before kernel init/resume is completed.
>>>>>>
>>>>>> Motivation to add these complications into i915 are not clear to 
>>>>>> me here. I mean there is no HuC on DG2 _yet_ is the premise of the 
>>>>>> series, right? So no backwards compatibility concerns. In this 
>>>>>> case why jump through the hoops and not let userspace handle all 
>>>>>> of this by just leaving the getparam return the true status?
>>>>>
>>>>> The main areas impacted by the fact that we can't guarantee that 
>>>>> HuC load is complete when i915 starts accepting submissions are 
>>>>> boot and suspend/resume, with the latter being the main problem; GT 
>>>>> reset is not a concern because HuC now survives it. A 
>>>>> suspend/resume can be transparent to userspace and therefore the 
>>>>> HuC status can temporarily flip from loaded to not without 
>>>>> userspace knowledge, especially if we start going into deeper 
>>>>> suspend states and start causing HuC resets when we go into runtime 
>>>>> suspend. Note that this is different from what happens during GT 
>>>>> reset for older platforms, because in that scenario we guarantee 
>>>>> that HuC reload is complete before we restart the submission 
>>>>> back-end, so userspace doesn't notice that the HuC status change. 
>>>>> We had an internal discussion about this problem with both media 
>>>>> and i915 archs and the conclusion was that the best option is for 
>>>>> i915 to stall media submission while HuC (re-)load is in progress.
>>>>
>>>> Resume is potentialy a good reason - I did not pick up on that from 
>>>> the cover letter. I read the statement about the unlikely and small 
>>>> window where HuC is not loaded during kernel init/resume and I guess 
>>>> did not pick up on the resume part.
>>>>
>>>> Waiting for GSC to load HuC from i915 resume is not an option?
>>>
>>> GSC is an aux device exported by i915, so AFAIU GSC resume can't 
>>> start until i915 resume completes.
>>
>> I'll dig into this in the next few days since I want to understand how 
>> exactly it works. Or someone can help explain.
>>
>> If in the end conclusion will be that i915 resume indeed cannot wait 
>> for GSC, then I think auto-blocking of queued up contexts on media 
>> engines indeed sounds unavoidable. Otherwise, as you explained, user 
>> experience post resume wouldn't be good.
> 
> Even if we could implement a wait, I'm not sure we should. GSC resume 
> and HuC reload takes ~300ms in most cases, I don't think we want to 
> block within the i915 resume path for that long.

Yeah maybe not. But entertaining the idea that it is technically 
possible to block - we could perhaps add uapi for userspace to mark 
contexts which want HuC access. Then track if there are any such 
contexts with outstanding submissions and only wait in resume if there 
are. If that would end up significantly less code on the i915 side to 
maintain is an open.

What would be the end result from users point of view in case where it 
suspended during video playback? The proposed solution from this series 
sees the video stuck after resume. Maybe compositor blocks as well since 
I am not sure how well they handle one window not providing new data. 
Probably depends on the compositor.

And then with a simpler solution definitely the whole resume would be 
delayed by 300ms.

With my ChromeOS hat the stalled media engines does sound like a better 
solution. But with the maintainer hat I'd like all options evaluated 
since there is attractiveness if a good enough solution can be achieved 
with significantly less kernel code.

You say 300ms is typical time for HuC load. How long it is on other 
platforms? If much faster then why is it so slow here?

>> However, do we really need to lie in the getparam? How about extend or 
>> add a new one to separate the loading vs loaded states? Since 
>> userspace does not support DG2 HuC yet this should be doable.
> 
> I don't really have a preference here. The media team asked us to do it 
> this way because they wouldn't have a use for the different "in 
> progress" and "done" states. If they're ok with having separate flags 
> that's fine by me.
> Tony, any feedback here?

We don't even have any docs in i915_drm.h in terms of what it means:

#define I915_PARAM_HUC_STATUS		 42

Seems to be a boolean. Status false vs true? Could you add some docs?

Regards,

Tvrtko

> 
> Thanks,
> Daniele
> 
>>
>>>> Will there be runtime suspend happening on the GSC device behind 
>>>> i915's back, or i915 and GSC will always be able to transition the 
>>>> states in tandem?
>>>
>>> They're always in sync. The GSC is part of the same HW PCI device as 
>>> the rest of the GPU, so they change HW state together.
>>
>> Okay thanks, I wasn't sure if it is the same or separate device.
>>
>> Regards,
>>
>> Tvrtko
>