Hello Qiang,
On 2024-06-26 08:49, Dragan Simic wrote:
On 2024-06-26 03:11, Qiang Yu wrote:
On Wed, Jun 26, 2024 at 2:15 AM Dragan Simic dsimic@manjaro.org wrote:
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.
Another option has become available for expressing additional module dependencies, weakdeps. [1][2] Long story short, weakdeps are similar to softdeps, in the sense of telling the initial ramdisk utilities to include additional kernel modules, but weakdeps result in no module loading being performed by userspace.
Maybe "weak" isn't the best possible word choice (arguably, "soft" also wasn't the best word choice), but weakdeps should be a better choice for use with Lima and governor_simpleondemand, because weakdeps provide the required information to the utilities used to generate initial ramdisk, while the actual module loading is left to the kernel.
The recent addition of weakdeps renders the previously mentioned harddeps obsolete, because weakdeps actually do what we need. Obviously, "weak" doesn't go along very well with the actual nature of the dependency between Lima and governor_simpleondemand, but it's pretty much just the somewhat unfortunate word choice.
The support for weakdeps has been already added to the kmod [3][4] and Dracut [5] userspace utilities. I'll hopefully add support for weakdeps to mkinitcpio [6] rather soon.
Maybe we could actually add MODULE_HARDDEP() as some kind of syntactic sugar, which would currently be an alias for MODULE_WEAKDEP(), so the actual hard module dependencies could be expressed properly, and possibly handled differently in the future, with no need to go back and track all such instances of hard module dependencies.
With all this in mind, here's what I'm going to do:
1) Submit a patch that adds MODULE_HARDDEP() as syntactic sugar 2) Implement support for weakdeps in Arch Linux's mkinitcpio [6] 3) Depending on what kind of feedback the MODULE_HARDDEP() patch receives, I'll submit follow-up patches for Lima and Panfrost, which will swap uses of MODULE_SOFTDEP() with MODULE_HARDDEP() or MODULE_WEAKDEP()
Looking forward to your thoughts.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/in... [2] https://lore.kernel.org/linux-kernel/20240724102349.430078-1-jtornosm@redhat... [3] https://github.com/kmod-project/kmod/commit/05828b4a6e9327a63ef94df544a042b5... [4] https://github.com/kmod-project/kmod/commit/d06712b51404061eef92cb275b830381... [5] https://github.com/dracut-ng/dracut-ng/commit/8517a6be5e20f4a6d87e55fce35ee3... [6] https://gitlab.archlinux.org/archlinux/mkinitcpio/mkinitcpio
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@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/?i... [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/b... [3] https://lore.kernel.org/dri-devel/4e1e00422a14db4e2a80870afb704405da16fd1b.1... [4] https://git.kernel.org/pub/scm/utils/kernel/kmod/kmod.git/commit/?id=49d8e0b...