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

[Dragan Simic <dsimic@manjaro.org>]

On 2024-07-03 15:20, Boris Brezillon wrote:
> On Wed, 03 Jul 2024 14:42:37 +0200
> Dragan Simic <dsimic@manjaro.org> 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@cknow.org>
>> > Cc: Furkan Kardame <f.kardame@manjaro.org>
>> > Cc: stable@vger.kernel.org
>> > Fixes: f3ba91228e8e ("drm/panfrost: Add initial panfrost driver")
>> > Signed-off-by: Dragan Simic <dsimic@manjaro.org>
>> 
>> Just checking, could this patch be accepted, please?
> 
> Yes, sorry for the delay. Here's my
> 
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

No worries, and thanks!

> Steve, any objection?
> 
>> The Lima
>> counterpart
>> has already been accepted. [6]
>> 
>> 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]
>> 
>> 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");