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

[Maxime Ripard]

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)?

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index c3c3f8c07258..b831a4227236 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -1459,12 +1459,15 @@ static void clk_core_init_rate_req(struct clk_core * const core,
 {
 	struct clk_core *parent;

-	if (WARN_ON(!core || !req))
+	if (WARN_ON(!req))
 		return;

 	memset(req, 0, sizeof(*req));
-
 	req->rate = rate;
+
+	if (!core)
+		return;
+
 	clk_core_get_boundaries(core, &req->min_rate, &req->max_rate);

 	parent = core->parent;

Thanks!
Maxime