Re: [PATCH v2] clk: mediatek: clk-mux: Add .determine_rate() callback

From: AngeloGioacchino Del Regno
Date: Wed Oct 12 2022 - 05:57:28 EST

Il 12/10/22 11:40, Maxime Ripard ha scritto:
On Wed, Oct 12, 2022 at 11:09:59AM +0200, AngeloGioacchino Del Regno wrote:
Il 12/10/22 10:55, Maxime Ripard ha scritto:
Hi,

On Tue, Oct 11, 2022 at 03:55:48PM +0200, AngeloGioacchino Del Regno wrote:
Since commit 262ca38f4b6e ("clk: Stop forwarding clk_rate_requests
to the parent"), the clk_rate_request is .. as the title says, not
forwarded anymore to the parent:

It's not entirely true, the rate request should still be forwarded, but
we don't pass the same instance of clk_rate_request anymore.

this produces an issue with the MediaTek clock MUX driver during GPU
DVFS on MT8195, but not on MT8192 or others.

This is because, differently from others, like MT8192 where all of
the clocks in the MFG parents tree are of mtk_mux type, but in the
parent tree of MT8195's MFG clock, we have one mtk_mux clock and
one (clk framework generic) mux clock, like so:

names: mfg_bg3d -> mfg_ck_fast_ref -> top_mfg_core_tmp (or) mfgpll
types: mtk_gate -> mux -> mtk_mux (or) mtk_pll

To solve this issue and also keep the GPU DVFS clocks code working
as expected, wire up a .determine_rate() callback for the mtk_mux
ops; for that, the standard clk_mux_determine_rate_flags() was used
as it was possible to.

It probably fixes things indeed, but I'm a bit worried that it just
works around the actual issue instead of fixing the actual bug...

This commit was successfully tested on MT6795 Xperia M5, MT8173 Elm,
MT8192 Spherion and MT8195 Tomato; no regressions were seen.

For the sake of some more documentation about this issue here's the
trace of it:

[ 12.211587] ------------[ cut here ]------------
[ 12.211589] WARNING: CPU: 6 PID: 78 at drivers/clk/clk.c:1462 clk_core_init_rate_req+0x84/0x90
[ 12.211593] Modules linked in: stp crct10dif_ce mtk_adsp_common llc rfkill snd_sof_xtensa_dsp
panfrost(+) sbs_battery cros_ec_lid_angle cros_ec_sensors snd_sof_of
cros_ec_sensors_core hid_multitouch cros_usbpd_logger snd_sof gpu_sched
snd_sof_utils fuse ipv6
[ 12.211614] CPU: 6 PID: 78 Comm: kworker/u16:2 Tainted: G W 6.0.0-next-20221011+ #58
[ 12.211616] Hardware name: Acer Tomato (rev2) board (DT)
[ 12.211617] Workqueue: devfreq_wq devfreq_monitor
[ 12.211620] pstate: 40400009 (nZcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 12.211622] pc : clk_core_init_rate_req+0x84/0x90
[ 12.211625] lr : clk_core_forward_rate_req+0xa4/0xe4
[ 12.211627] sp : ffff80000893b8e0
[ 12.211628] x29: ffff80000893b8e0 x28: ffffdddf92f9b000 x27: ffff46a2c0e8bc05
[ 12.211632] x26: ffff46a2c1041200 x25: 0000000000000000 x24: 00000000173eed80
[ 12.211636] x23: ffff80000893b9c0 x22: ffff80000893b940 x21: 0000000000000000
[ 12.211641] x20: ffff46a2c1039f00 x19: ffff46a2c1039f00 x18: 0000000000000000
[ 12.211645] x17: 0000000000000038 x16: 000000000000d904 x15: 0000000000000003
[ 12.211649] x14: ffffdddf9357ce48 x13: ffffdddf935e71c8 x12: 000000000004803c
[ 12.211653] x11: 00000000a867d7ad x10: 00000000a867d7ad x9 : ffffdddf90c28df4
[ 12.211657] x8 : ffffdddf9357a980 x7 : 0000000000000000 x6 : 0000000000000004
[ 12.211661] x5 : ffffffffffffffc8 x4 : 00000000173eed80 x3 : ffff80000893b940
[ 12.211665] x2 : 00000000173eed80 x1 : ffff80000893b940 x0 : 0000000000000000
[ 12.211669] Call trace:
[ 12.211670] clk_core_init_rate_req+0x84/0x90
[ 12.211673] clk_core_round_rate_nolock+0xe8/0x10c
[ 12.211675] clk_mux_determine_rate_flags+0x174/0x1f0
[ 12.211677] clk_mux_determine_rate+0x1c/0x30
[ 12.211680] clk_core_determine_round_nolock+0x74/0x130
[ 12.211682] clk_core_round_rate_nolock+0x58/0x10c
[ 12.211684] clk_core_round_rate_nolock+0xf4/0x10c
[ 12.211686] clk_core_set_rate_nolock+0x194/0x2ac
[ 12.211688] clk_set_rate+0x40/0x94
[ 12.211691] _opp_config_clk_single+0x38/0xa0
[ 12.211693] _set_opp+0x1b0/0x500
[ 12.211695] dev_pm_opp_set_rate+0x120/0x290
[ 12.211697] panfrost_devfreq_target+0x3c/0x50 [panfrost]
[ 12.211705] devfreq_set_target+0x8c/0x2d0
[ 12.211707] devfreq_update_target+0xcc/0xf4
[ 12.211708] devfreq_monitor+0x40/0x1d0
[ 12.211710] process_one_work+0x294/0x664
[ 12.211712] worker_thread+0x7c/0x45c
[ 12.211713] kthread+0x104/0x110
[ 12.211716] ret_from_fork+0x10/0x20
[ 12.211718] irq event stamp: 7102
[ 12.211719] hardirqs last enabled at (7101): [<ffffdddf904ea5a0>] finish_task_switch.isra.0+0xec/0x2f0
[ 12.211723] hardirqs last disabled at (7102): [<ffffdddf91794b74>] el1_dbg+0x24/0x90
[ 12.211726] softirqs last enabled at (6716): [<ffffdddf90410be4>] __do_softirq+0x414/0x588
[ 12.211728] softirqs last disabled at (6507): [<ffffdddf904171d8>] ____do_softirq+0x18/0x24
[ 12.211730] ---[ end trace 0000000000000000 ]---

... Indeed, you shouldn't hit that warning at all. It happens in
clk_core_round_rate_nolock, which takes (before your patch) the
CLK_SET_RATE_PARENT branch. This indeed has been changed by the patch
you mentioned, and will call clk_core_forward_rate_req() now, that in
turn calls clk_core_init_rate_nolock().

I think the warning you hit is because core->parent is NULL, which is
passed to clk_core_forward_rate_req() as the parent argument, and we'll
call clk_core_init_rate_req() with parent set as the core argument.

In clk_core_init_rate_req(), the first thing we do is a WARN_ON(!core),
which is what you hit here I think.

This is different to the previous behavior that was calling
clk_core_round_rate_nolock() with core->parent directly, and
clk_core_round_rate_nolock() if its core argument is NULL will set
req->rate to 0 and bail out without returning an error.

Now, your patch probably works because now that you provide a
determine_rate implementation, clk_core_can_round() returns true and
you'll take a different branch in clk_core_round_rate_nolock(), avoiding
that issue entirely.

Does that patch work better (on top of next-20221012)?

I admit I didn't go too deep in the research, as my brain processed that as
"this is a mux clock, not really different from a standard mux, this callback
is missing, that's not optimal"... then that fixed it and called it a day.

I should've prolonged my research for a better understanding of what was
actually going on.

No worries :)

What you said actually opened my mind and, with little surprise, your patch
works as good as mine - no warnings and the clock scales as expected!

I'm actually wondering if you didn't encounter two issues. What kernel
were you testing before? If it's older than today's next
(next-20221012), you're likely missing


I was testing next-20221011.

https://lore.kernel.org/linux-clk/20221010-rpi-clk-fixes-again-v1-0-d87ba82ac404@xxxxxxxxxx/

Which is likely to be what fixed the clock scaling. And my patch only
fixed the warning. Could you test next-20221012? If I'm right, you
should only get the warning.


No, I am getting the same situation even after rebasing over next-20221012, without
any of the two patches (determine_rate() for mtk mux, nor the one you shared for
clk.c), when the warning happens, I get very slow GPU operation and the same "nice"
timeout:

[ 27.785514] panfrost 13000000.gpu: gpu sched timeout, js=1, config=0x7b00, status=0x0, head=0xa1cb180, tail=0xa1cb180, sched_job=00000000f07d39e3

...so I'm not encountering the same issue that you've fixed with the patches that
got merged in next-20221012.

Of course, as you were expecting, the warning is also still there and still
the same:

[ 27.750504] WARNING: CPU: 4 PID: 164 at drivers/clk/clk.c:1462 clk_core_init_rate_req+0x84/0x90

Cheers,
Angelo

I still think that the mtk-mux driver should get a determine_rate callback but,
at this point, that's going to have an entirely different commit description...

Yeah, it might, but as you said it's a separate discussion

Please go on and send your patch: if you want, please remember to add me to
the Cc's, so that I can give you my R-b tag in a timely manner.

Thanks!
Maxime