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

From: Martin Peres <martin.peres@free.fr>
To: Jason Ekstrand <jason@jlekstrand.net>,
	Matthew Brost <matthew.brost@intel.com>,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org
Cc: tvrtko.ursulin@intel.com, jason.ekstrand@intel.com,
	daniele.ceraolospurio@intel.com, jon.bloomfield@intel.com,
	daniel.vetter@intel.com, john.c.harrison@intel.com
Subject: Re: [RFC PATCH 00/97] Basic GuC submission support in the i915
Date: Mon, 10 May 2021 16:55:34 +0300	[thread overview]
Message-ID: <546e4ee4-9aa4-1967-cdcb-a561cac582ef@free.fr> (raw)
In-Reply-To: <17953669798.2817.c6988b7ea6112e3e892765a0d4287e0c@jlekstrand.net>

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

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.

> 
>> 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.

> 
>> 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