Date: Wed, 26 Jun 2024 08:49:09 +0200
From: Dragan Simic <dsimic@...jaro.org>
To: Qiang Yu <yuq825@...il.com>
Cc: Maxime Ripard <mripard@...nel.org>, dri-devel@...ts.freedesktop.org,
 lima@...ts.freedesktop.org, maarten.lankhorst@...ux.intel.com,
 tzimmermann@...e.de, airlied@...il.com, daniel@...ll.ch,
 linux-kernel@...r.kernel.org, Philip Muller <philm@...jaro.org>, Oliver
 Smith <ollieparanoid@...tmarketos.org>, Daniel Smith <danct12@...root.org>,
 stable@...r.kernel.org
Subject: Re: [PATCH] drm/lima: Mark simple_ondemand governor as softdep

Hello Qiang,

On 2024-06-26 03:11, Qiang Yu wrote:
> On Wed, Jun 26, 2024 at 2:15 AM Dragan Simic <dsimic@...jaro.org> 
> wrote:
>> 
>> Hello everyone,
>> 
>> Just checking, any further thoughts about this patch?
>> 
> I'm OK with this as a temp workaround because it's simple and do no 
> harm
> even it's not perfect. If no other better suggestion for short term, 
> I'll submit
> this at weekend.

Thanks.  Just as you described it, it's far from perfect, but it's still
fine until there's a better solution, such as harddeps.  I'll continue 
my
research about the possibility for adding harddeps, which would 
hopefully
replace quite a few instances of the softdep (ab)use.

>> On 2024-06-18 21:22, Dragan Simic wrote:
>> > On 2024-06-18 12:33, Dragan Simic wrote:
>> >> 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@...il.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

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