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

[Steven Price <steven.price@arm.com>]

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

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

> 
> 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,

Steve

> 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");