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/?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...