Re: [RFC PATCH 00/97] Basic GuC submission support in the i915

[Jason Ekstrand <jason@xxxxxxxxxxxxxx>]

On May 10, 2021 08:55:55 Martin Peres <martin.peres@xxxxxxx> wrote:

On 10/05/2021 02:11, Jason Ekstrand wrote:

On May 9, 2021 12:12:36 Martin Peres <martin.peres@xxxxxxx> wrote:

Hi,

On 06/05/2021 22:13, Matthew Brost wrote:

Basic GuC submission support. This is the first bullet point in the

upstreaming plan covered in the following RFC [1].

At a very high level the GuC is a piece of firmware which sits between

the i915 and the GPU. It offloads some of the scheduling of contexts

from the i915 and programs the GPU to submit contexts. The i915

communicates with the GuC and the GuC communicates with the GPU.

May I ask what will GuC command submission do that execlist won't/can't

do? And what would be the impact on users? Even forgetting the troubled

history of GuC (instability, performance regression, poor level of user

support, 6+ years of trying to upstream it...), adding this much code

and doubling the amount of validation needed should come with a

rationale making it feel worth it... and I am not seeing here. Would you

mind providing the rationale behind this work?

GuC submission will be disabled by default on all current upstream

platforms behind a module parameter - enable_guc. A value of 3 will

enable submission and HuC loading via the GuC. GuC submission should

work on all gen11+ platforms assuming the GuC firmware is present.

What is the plan here when it comes to keeping support for execlist? I

am afraid that landing GuC support in Linux is the first step towards

killing the execlist, which would force users to use proprietary

firmwares that even most Intel engineers have little influence over.

Indeed, if "drm/i915/guc: Disable semaphores when using GuC scheduling"

which states "Disable semaphores when using GuC scheduling as semaphores

are broken in the current GuC firmware." is anything to go by, it means

that even Intel developers seem to prefer working around the GuC

firmware, rather than fixing it.

Yes, landing GuC support may be the first step in removing execlist 

support. The inevitable reality is that GPU scheduling is coming and 

likely to be there only path in the not-too-distant future. (See also 

the ongoing thread with AMD about fences.) I'm not going to pass 

judgement on whether or not this is a good thing.  I'm just reading the 

winds and, in my view, this is where things are headed for good or ill.

In answer to the question above, the answer to "what do we gain from 

GuC?" may soon be, "you get to use your GPU."  We're not there yet and, 

again, I'm not necessarily advocating for it, but that is likely where 

things are headed.

This will be a sad day, especially since it seems fundamentally opposed 

with any long-term support, on top of taking away user freedom to 

fix/tweak their system when Intel won't.

A firmware-based submission model isn't a bad design IMO and, aside from 

the firmware freedom issues, I think there are actual advantages to the 

model. Immediately, it'll unlock a few features like parallel submission 

(more on that in a bit) and long-running compute because they're 

implemented in GuC and the work to implement them properly in the 

execlist scheduler is highly non-trivial. Longer term, it may (no 

guarantees) unlock some performance by getting the kernel out of the way.

Oh, I definitely agree with firmware-based submission model not being a 

bad design. I was even cheering for it in 2015. Experience with it made 

me regret that deeply since :s

But with the DRM scheduler being responsible for most things, I fail to 

see what we could offload in the GuC except context switching (like 

every other manufacturer). The problem is, the GuC does way more than 

just switching registers in bulk, and if the number of revisions of the 

GuC is anything to go by, it is way too complex for me to feel 

comfortable with it.

It's more than just bulk register writes. When it comes to load-balancing multiple GPU users, firmware can theoretically preempt and switch faster leading to more efficient time-slicing. All we really need the DRM scheduler for is handling implicit dma_fence dependencies between different applications.

In the same vein, I have another concern related to the impact of GuC on

Linux's stable releases. Let's say that in 3 years, a new application

triggers a bug in command submission inside the firmware. Given that the

Linux community cannot patch the GuC firmware, how likely is it that

Intel would release a new GuC version? That would not be necessarily

such a big problem if newer versions of the GuC could easily be

backported to this potentially-decade-old Linux version, but given that

the GuC seems to have ABI-breaking changes on a monthly cadence (we are

at major version 60 *already*? :o), I would say that it is

highly-unlikely that it would not require potentially-extensive changes

to i915 to make it work, making the fix almost impossible to land in the

stable tree... Do you have a plan to mitigate this problem?

Patches like "drm/i915/guc: Disable bonding extension with GuC

submission" also make me twitch, as this means the two command

submission paths will not be functionally equivalent, and enabling GuC

could thus introduce a user-visible regression (one app used to work,

then stopped working). Could you add in the commit's message a proof

that this would not end up being a user regression (in which case, why

have this codepath to begin with?).

I'd like to address this one specifically as it's become something of a 

speciality of mine the past few weeks. The current bonded submission 

model is bad. It provides a plethora of ways for a client to back itself 

into a corner and doesn't actually provide the guarantees the media 

driver needs for its real-time high-resolution decode. It's bad enough 

we're seriously considering ripping it out, backwards compatibility or 

not. The good news is that very little that your average desktop user 

does depends on it: basically just real-time >4K video decode.

The new parallel submit API is much better and should be the path 

forward. (We should have landed parallel submit the first time around.) 

It isn't full of corners and does let us provides actual parallel 

execution guarantees. It also gives the scheduler the information it 

needs to reliably provide those guarantees. >

If we need to support the parallel submit API with the execlist 

back-end, that's totally possible. The choice to only implement the 

parallel submit API with GuC is a pragmatic one. We're trying to get 

upstream back on it's feet and get all the various up-and-coming bits of 

hardware enabled. Enabling the new API in the execlist back-end makes 

that pipeline longer.

I feel your pain, and wish you all the best to get GEM less complex

and more manageable.

So, if I understood correctly, the plan is just to regress 4K+ video 

decoding for people who do not enable GuC scheduling, or did not also 

update to a recent-enough media driver that would support this new 

interface? If it is indeed only for over 4K videos, then whatever. If it 

is 4K, it starts being a little bad, assuming graceful fallback to 

CPU-based decoding. What's the test plan for this patch then? The patch 

in its current form is definitely not making me confident.

My understanding is that it's only >4k that's affected; we've got enough bandwidth on a single VCS for 4K. I'm not sure where the exact cut-off is (it may be a little higher than 4k) but real-time 4k should be fine and real-time 8k requires parallel submit. So we're really not cutting off many use-cases. Also, as I said above, the new API can be implemented with the execlist scheduler if needed. We've just pragmatically deprioritized it.

--Jason

Finally, could you explain why IGT tests need to be modified to work the

GuC [1], and how much of the code in this series is covered by

existing/upcoming tests? I would expect a very solid set of tests to

minimize the maintenance burden, and enable users to reproduce potential

issues found in this new codepath (too many users run with enable_guc=3,

as can be seen on Google[2]).

The IGT changes, as I understand them, are entirely around switching to 

the new parallel submit API. There shouldn't be a major effect to most 

users.

Right, this part I followed, but failed to connect it to the GuC... 

because I couldn't see why it would be needed (execlist requiring a lot 

more work).

I sincerely wish for the GuC to stay away from upstream because of the 

above concerns (which are yet to be addressed), but if Intel were to 

push forward with the plan to drop execlist, I can foresee a world of 

trouble for users... That is of course unless the GuC were to be open 

sourced, with people outside of Intel able to sign their own builds or 

run unsigned. Failing that, let's hope the last 6 years were just a bad 

start, and the rapid climb in major version of the GuC will magically 

stop! I hope execlists will remain at feature parity with the GuC when 

possible... but deplore the increase in validation needs which will only 

hurt users in the end.

Thanks for your honest answer,

Martin

--Jason

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