Re: [PATCH] drm/lima: Mark simple_ondemand governor as softdep

[Dragan Simic <dsimic@manjaro.org>]

On 2024-06-18 12:33, Dragan Simic wrote:
> Hello Qiang and Maxime,
> 
> On 2024-06-18 10:13, Maxime Ripard wrote:
>> On Tue, Jun 18, 2024 at 04:01:26PM GMT, Qiang Yu wrote:
>>> On Tue, Jun 18, 2024 at 12:33 PM Qiang Yu <yuq825@gmail.com> wrote:
>>> >
>>> > I see the problem that initramfs need to build a module dependency chain,
>>> > but lima does not call any symbol from simpleondemand governor module.
>>> > softdep module seems to be optional while our dependency is hard one,
>>> > can we just add MODULE_INFO(depends, _depends), or create a new
>>> > macro called MODULE_DEPENDS()?
> 
> I had the same thoughts, because softdeps are for optional module
> dependencies, while in this case it's a hard dependency.  Though,
> I went with adding a softdep, simply because I saw no better option
> available.
> 
>>> This doesn't work on my side because depmod generates modules.dep
>>> by symbol lookup instead of modinfo section. So softdep may be our 
>>> only
>>> choice to add module dependency manually. I can accept the softdep
>>> first, then make PM optional later.
> 
> I also thought about making devfreq optional in the Lima driver,
> which would make this additional softdep much more appropriate.
> Though, I'm not really sure that's a good approach, because not
> having working devfreq for Lima might actually cause issues on
> some devices, such as increased power consumption.
> 
> In other words, it might be better to have Lima probing fail if
> devfreq can't be initialized, rather than having probing succeed
> with no working devfreq.  Basically, failed probing is obvious,
> while a warning in the kernel log about no devfreq might easily
> be overlooked, causing regressions on some devices.
> 
>> It's still super fragile, and depends on the user not changing the
>> policy. It should be solved in some other, more robust way.
> 
> I see, but I'm not really sure how to make it more robust?  In
> the end, some user can blacklist the simple_ondemand governor
> module, and we can't do much about it.
> 
> Introducing harddeps alongside softdeps would make sense from
> the design standpoint, but the amount of required changes wouldn't
> be trivial at all, on various levels.

After further investigation, it seems that the softdeps have
already seen a fair amount of abuse for what they actually aren't
intended, i.e. resolving hard dependencies.  For example, have
a look at the commit d5178578bcd4 (btrfs: directly call into
crypto framework for checksumming) [1] and the lines containing
MODULE_SOFTDEP() at the very end of fs/btrfs/super.c. [2]

If a filesystem driver can rely on the abuse of softdeps, which
admittedly are a bit fragile, I think we can follow the same
approach, at least for now.

With all that in mind, I think that accepting this patch, as well
as the related Panfrost patch, [3] should be warranted.  I'd keep
investigating the possibility of introducing harddeps in form
of MODULE_HARDDEP() and the related support in kmod project,
similar to the already existing softdep support, [4] but that
will inevitably take a lot of time, both for implementing it
and for reaching various Linux distributions, which is another
reason why accepting these patches seems reasonable.

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d5178578bcd4
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/btrfs/super.c#n2593
[3] 
https://lore.kernel.org/dri-devel/4e1e00422a14db4e2a80870afb704405da16fd1b.1718655077.git.dsimic@manjaro.org/
[4] 
https://git.kernel.org/pub/scm/utils/kernel/kmod/kmod.git/commit/?id=49d8e0b59052999de577ab732b719cfbeb89504d