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

From: Dragan Simic
Date: Thu Jul 25 2024 - 04:24:24 EST

Next message: Chris Li: "Re: [PATCH v4 2/3] mm: swap: mTHP allocate swap entries from nonfull list"
Previous message: Dragan Simic: "Re: [PATCH] drm/lima: Mark simple_ondemand governor as softdep"
Next in thread: Steven Price: "Re: [PATCH] drm/panfrost: Mark simple_ondemand governor as softdep"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

Hello Steven and Boris,

On 2024-07-03 16:52, Dragan Simic wrote:

On 2024-07-03 15:20, Steven Price wrote:

On 03/07/2024 13:42, Dragan Simic wrote:

On 2024-06-17 22:17, Dragan Simic wrote:

Panfrost DRM driver uses devfreq to perform DVFS, while using
simple_ondemand
devfreq governor by default. This causes driver initialization to
fail on
boot when simple_ondemand governor isn't built into the kernel
statically,
as a result of the missing module dependency and, consequently, the
required
governor module not being included in the initial ramdisk. Thus,
let's mark
simple_ondemand governor as a softdep for Panfrost, to have its kernel
module
included in the initial ramdisk.

This is a rather longstanding issue that has forced distributions to
build
devfreq governors statically into their kernels, [1][2] or has forced
users
to introduce some unnecessary workarounds. [3]

For future reference, not having support for the simple_ondemand
governor in
the initial ramdisk produces errors in the kernel log similar to these
below,
which were taken from a Pine64 RockPro64:

panfrost ff9a0000.gpu: [drm:panfrost_devfreq_init [panfrost]]
*ERROR* Couldn't initialize GPU devfreq
panfrost ff9a0000.gpu: Fatal error during GPU init
panfrost: probe of ff9a0000.gpu failed with error -22

Having simple_ondemand marked as a softdep for Panfrost may not
resolve this
issue for all Linux distributions. In particular, it will remain
unresolved
for the distributions whose utilities for the initial ramdisk
generation do
not handle the available softdep information [4] properly yet.
However, some
Linux distributions already handle softdeps properly while generating
their
initial ramdisks, [5] and this is a prerequisite step in the right
direction
for the distributions that don't handle them properly yet.

[1] https://gitlab.manjaro.org/manjaro-arm/packages/core/linux/-/blob/linux61/config?ref_type=heads#L8180
[2] https://salsa.debian.org/kernel-team/linux/-/merge_requests/1066
[3] https://forum.pine64.org/showthread.php?tid=15458
[4] https://git.kernel.org/pub/scm/utils/kernel/kmod/kmod.git/commit/?id=49d8e0b59052999de577ab732b719cfbeb89504d
[5] https://github.com/archlinux/mkinitcpio/commit/97ac4d37aae084a050be512f6d8f4489054668ad

Cc: Diederik de Haas <didi.debian@xxxxxxxxx>
Cc: Furkan Kardame <f.kardame@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: f3ba91228e8e ("drm/panfrost: Add initial panfrost driver")
Signed-off-by: Dragan Simic <dsimic@xxxxxxxxxxx>

Reviewed-by: Steven Price <steven.price@xxxxxxx>

Thanks!

Just checking, could this patch be accepted, please? The Lima counterpart
has already been accepted. [6]

Thanks for the prod - I have to admit I saw there was discussion about
the Lima patch and so just put this on my list to look again later after
the discussion had reached a conclusion.

The approach in this patch is 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 introducing harddeps, which would hopefully
replace quite a few instances of the softdep (ab)use that already extend
rather far. For example, have a look at the commit d5178578bcd4 (btrfs:
directly call into crypto framework for checksumming) [7] and the lines
containing MODULE_SOFTDEP() at the very end of fs/btrfs/super.c. [8]

I agree - it's not perfect, but it's the best we have for now. I hope
sometime we'll have a cleaner solution to express dependencies like this
(good luck! ;) ).

Thanks. :) Implementing harddeps is _relatively_ straightforward, but
getting full support for harddeps reach various Linux distributions is
going to be an uphill battle without doubt. :)

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 Panfrost and governor_simpleondemand, because weakdeps provide
the required information to the utilities used to generate initial ramdisks,
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
Panfrost 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/include/linux/module.h?id=61842868de13aa7fd7391c626e889f4d6f1450bf
[2] https://lore.kernel.org/linux-kernel/20240724102349.430078-1-jtornosm@xxxxxxxxxx/T/#u
[3] https://github.com/kmod-project/kmod/commit/05828b4a6e9327a63ef94df544a042b5e9ce4fe7
[4] https://github.com/kmod-project/kmod/commit/d06712b51404061eef92cb275b8303814fca86ec
[5] https://github.com/dracut-ng/dracut-ng/commit/8517a6be5e20f4a6d87e55fce35ee3e29e2a1150
[6] https://gitlab.archlinux.org/archlinux/mkinitcpio/mkinitcpio

If a filesystem driver can rely on the (ab)use of softdeps, which may be
fragile or seen as a bit wrong, I think we can follow the same approach,
at least until a better solution is available.

[6] https://cgit.freedesktop.org/drm/drm-misc/commit/?id=0c94f58cef319ad054fd909b3bf4b7d09c03e11c
[7] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d5178578bcd4
[8] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/btrfs/super.c#n2593

---
drivers/gpu/drm/panfrost/panfrost_drv.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c
b/drivers/gpu/drm/panfrost/panfrost_drv.c
index ef9f6c0716d5..149737d7a07e 100644
--- a/drivers/gpu/drm/panfrost/panfrost_drv.c
+++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
@@ -828,3 +828,4 @@ module_platform_driver(panfrost_driver);
MODULE_AUTHOR("Panfrost Project Developers");
MODULE_DESCRIPTION("Panfrost DRM Driver");
MODULE_LICENSE("GPL v2");
+MODULE_SOFTDEP("pre: governor_simpleondemand");

Next message: Chris Li: "Re: [PATCH v4 2/3] mm: swap: mTHP allocate swap entries from nonfull list"
Previous message: Dragan Simic: "Re: [PATCH] drm/lima: Mark simple_ondemand governor as softdep"
Next in thread: Steven Price: "Re: [PATCH] drm/panfrost: Mark simple_ondemand governor as softdep"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]