[PATCH v2 13/15] drm/panthor: Allow driver compilation

[Steven Price]

On 21/08/2023 18:56, Robin Murphy wrote:
> On 2023-08-14 12:18, Steven Price wrote:
>> On 11/08/2023 20:26, Robin Murphy wrote:
>>> On 2023-08-11 17:56, Daniel Stone wrote:
>>>> Hi,
>>>>
>>>> On 11/08/2023 17:35, Robin Murphy wrote:
>>>>> On 2023-08-09 17:53, Boris Brezillon wrote:
>>>>>> +obj-$(CONFIG_DRM_PANTHOR) += panthor.o
>>>>>
>>>>> FWIW I still think it would be nice to have a minor
>>>>> directory/Kconfig/Makefile reshuffle and a trivial bit of extra
>>>>> registration glue to build both drivers into a single module. It
>>>>> seems like it could be a perpetual source of confusion to end users
>>>>> where Mesa "panfrost" is the right option but kernel "panfrost" is
>>>>> the wrong one. Especially when pretty much every other GPU driver is
>>>>> also just one big top-level module to load for many different
>>>>> generations of hardware. Plus it would mean that if someone did want
>>>>> to have a go at deduplicating the resource-wrangling boilerplate for
>>>>> OPPs etc. in future, there's more chance of being able to do so
>>>>> meaningfully.
>>>>
>>>> It might be nice to point it out, but to be fair Intel and AMD both
>>>> have two (or more) drivers, as does Broadcom/RPi. As does, err ...
>>>> Mali.
>>>
>>> Indeed, I didn't mean to imply that I'm not aware that e.g. gma500 is to
>>> i915 what lima is to panfrost. It was more that unlike the others where
>>> there's a pretty clear line in the sand between "driver for old
>>> hardware" and "driver for the majority of recent hardware", this one
>>> happens to fall splat in the middle of the current major generation such
>>> that panfrost is the correct module for Mali Bifrost but also the wrong
>>> one for Mali Bifrost... :/
>>
>> Well panfrost.ko is the correct module for all Bifrost ;) It's Valhall
>> that's the confusing one.
> 
> Bah, you see? If even developers sufficiently involved to be CCed on the
> patches can't remember what's what, what hope does Joe User have? :D
> 
>> I would hope that for most users they can just build both panfrost and
>> panthor and everything will "Just Work (tm)". I'm not sure how much
>> users are actually aware of the architecture family of their GPU.
>>
>> I think at the moment (until marketing mess it up) there's also the
>> 'simple' rule:
>>
>> * Mali T* is Midgard and supported by panfrost.ko
>> * Mali Gxx (two digits) is Bifrost or first-generation Valhall and
>> supported by panfrost.ko
>> * Mali Gxxx (three digits) is Valhall CSF and supported by panthor.
>>
>> (and Immortalis is always three digits and Valhall CSF).
> 
> With brain now engaged, indeed that sounds right. However if the
> expectation is that most people would steer clear even of marketing's
> alphabet soup and just enable everything, that could also be seen as
> somewhat of an argument for just putting it all together and not
> bothering with a separate option.
> 
>>>> I can see the point, but otoh if someone's managed to build all the
>>>> right regulator/clock/etc modules to get a working system, they'll
>>>> probably manage to figure teh GPU side out?
>>>
>>> Maybe; either way I guess it's not really my concern, since I'm the only
>>> user that *I* have to support, and I do already understand it. From the
>>> upstream perspective I mostly just want to hold on to the hope of not
>>> having to write my io-pgtable bugs twice over if at all possible :)
>>
>> I agree it would be nice to merge some of the common code, I'm hoping
>> this is something that might be possible in the future. But at the
>> moment the focus is on trying to get basic support for the new GPUs
>> without the danger of regressing the old GPUs.
> 
> Yup, I get that, it's just the niggling concern I have is whether what
> we do at the moment might paint us into a corner with respect to what
> we're then able to change later; I know KConfig symbols are explicitly
> not ABI, but module names and driver names might be more of a grey area.
> 
>> And, to be honest, for a fair bit of the common code in
>> panfrost/panthorm it's common to a few other drivers too. So the correct
>> answer might well be to try to add more generic helpers (devfreq,
>> clocks, power domains all spring to mind - there's a lot of boiler plate
>> and nothing very special about Mali).
> 
> That much is true, however I guess there's also stuff like perf counter
> support which is less likely to be DRM-level generic but perhaps still
> sufficiently similar between JM and CSF. The main thing I don't know,
> and thus feel compelled to poke at, is whether there's any possibility
> that once the new UAPI is mature, it might eventually become preferable
> to move Job Manager support over to some subset of that rather than
> maintain two whole UAPIs in parallel (particularly at the Mesa end). My
> (limited) understanding is that all the BO-wrangling and MMU code is
> primarily different here for the sake of supporting new shiny UAPI
> features, not because of anything inherent to CSF itself (other than CSF
> being the thing which makes supporting said features feasible). If
> that's a preposterous idea and absolutely never ever going to be
> realistic, then fine, but if not, then it feels like the kind of thing
> that my all-too-great experience of technical debt and bad short-term
> decisions tells me is worth planning around from the very start.

I agree this seems to be more a "political" decision rather than a
technical one. There is an attempt to start supporting Mali CSF GPUs
better and hopefully have more engagement from within Arm as well as Arm
backing Collabora[1]. This means there's some desire to be able to work
on panthor without having to worry about the potential of regressing
panfrost.

But CSF also provides some fairly radical changes to the way the GPU is
driven: firmware scheduling being the obvious one, and user-mode
submission being something that is hopefully coming soon. So to some
extent there's going to be two UAPIs because the GPU interface has changed.

However, there are definitely aspects of panthor that could apply to
panfrost - VM_BIND *could* be implemented for panfrost and potentially
could be useful. And the control of the GPU's VA space that panthor
provides is something that's lacking in panfrost. The question that I
see is, if panfrost was extended to include these APIs, would anyone use
them? If no-one is going to work on the Mesa side to make use of these
features in panfrost then it's likely to be untested (buggy) code; we'd
be relying on it "being the same as CSF" while not quite being.

In terms of the question of one kernel module or two: it's a good
question. There's a patch that moves panfrost over to using drm_exec[2]
which requires loading a new kernel module - it broke my test setup, but
I don't think we generally consider this ABI that we mustn't break. So I
think there is scope for changing our minds in the future if necessary.

Given that the two drivers are currently not at all combined it seems
sensible to me to build separate kernel modules, but I've no strong
views on that. And it might make sharing code in the future harder.

Steve

[1]
https://www.arm.com/company/news/2023/07/arm-expands-open-source-partnerships

[2]
https://lore.kernel.org/r/20230712124704.333004-6-christian.koenig%40amd.com