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

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.

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.

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.

--Jason