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

Maxime Ripard
2022-10-12T16:42:27Z

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On Wed, Oct 12, 2022 at 03:56:19PM +0200, Maxime Ripard wrote:
> On Wed, Oct 12, 2022 at 02:14:39PM +0200, AngeloGioacchino Del Regno wrote:
> > Il 12/10/22 13:48, Maxime Ripard ha scritto:
> > > On Wed, Oct 12, 2022 at 11:57:15AM +0200, AngeloGioacchino Del Regno wrote:
> > > > 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@cerno.tech/
> > > > >
> > > > > 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
> > >
> > > Ok. I'm still a bit unsure why it actually fixes anything.
> > >
> > > In the current next, clk_core_init_rate_req (through
> > > clk_core_forward_rate_req) will bail out right away, but the patch will
> > > clear the request and set the requested rate.
> > >
> > > I don't think the req->rate assignment changes anything because our next
> > > call will be to clk_core_round_rate_nolock that will clear it in the
> > > !core path, but it might just be that we don't initialize the request
> > > and end up with some garbage data in it?
> > >
> > > Could you test, on top of next-20221012,
> >
> > No that's not the case, this patch has no effect at all (yes I've tested it).
>
> Ok. Too bad. I've tried to build a test case that was reproducing what
> you experience, but I don't seem to be able to build one for now.
>
> If we go back to your previous explanation on the clock tree, and what
> we discussed then, there's still something a bit puzzling.
>
> You were saying that are calling clk_set_rate on mfg_bg3d, which is a
> mtk_gate, registered with mtk_clk_register_gate(), and with the
> mtk_clk_gate_ops_setclr clk_ops. There's no determine_rate / round_rate
> implementation which makes sense for a gate, and CLK_SET_RATE_PARENT is
> set by mtk_clk_register_gate(). Everything's good.
>
> The clk_set_rate call will thus be forwarded to its parent,
> mfg_ck_fast_ref, which is a mux, registered with
> devm_clk_hw_register_mux(), with CLK_SET_RATE_PARENT as well. Makes
> sense too, any clk_set_rate() call will thus evaluate all of the
> mfg_ck_fast_ref parents and will either adjust the rate of a parent, or
> reparent.
>
> mfg_ck_fast_ref then has two parents, top_mfg_core_tmp and mfgpll.
> Judging from your patch, top_mfg_core_tmp is being used.
> top_mfg_core_tmp is a mtk_mux, registered by mtk_clk_register_mux()
> (through mtk_clk_register_muxes()), and using
> mtk_mux_gate_clr_set_upd_ops. CLK_SET_RATE_PARENT is set by
> mtk_clk_register_mux(). That flag still makes sense for a mux. However,
> it's really not clear to me how a mux operates without a determine_rate
> implementation to forward the rates to each of its parents.
>
> It looks like we were relying before on the fact that the request was
> indeed forwarded as is to the current parent. It looks like the parent
> was not registered at all (core->parent being NULL), and then it was
> working because of $REASON, possibly one of the muxes preferred to
> switch to some other clock source.
>
> I think we need to address this in multiple ways:
>
> - If you have ftrace enabled on that platform, running with "tp_printk
> trace_event=clk:*" in the kernel command line on a failing kernel would
> be great, so that we can figure out what is happening exactly.

I spent more time trying to create a setup similar to yours but couldn't
find anything obviously wrong. In addition to the above, could you start
a faulty kernel with that patch:

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 884a58a98b6b..774953901a35 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -262,6 +262,9 @@ EXPORT_SYMBOL_GPL(__clk_get_name);

const char *clk_hw_get_name(const struct clk_hw *hw)
{
+ if (!hw || !hw->core)
+ return "(null)";
+
return hw->core->name;
}
EXPORT_SYMBOL_GPL(clk_hw_get_name);
@@ -1471,6 +1474,17 @@ static bool clk_core_can_round(struct clk_core * const core)
return core->ops->determine_rate || core->ops->round_rate;
}

+static void clk_core_request_dump(const struct clk_core *core,
+ const struct clk_rate_request *req)
+{
+ pr_crit("%s\n", core ? core->name : "(null)");
+ pr_crit("\trate %lu\n", req->rate);
+ pr_crit("\tmin %lu, max %lu\n", req->min_rate, req->max_rate);
+ pr_crit("\tbest parent %s, rate %lu\n",
+ clk_hw_get_name(req->best_parent_hw),
+ req->best_parent_rate);
+}
+
static int clk_core_round_rate_nolock(struct clk_core *core,
struct clk_rate_request *req)
{
@@ -2254,8 +2268,12 @@ static unsigned long clk_core_req_round_rate_nolock(struct clk_core *core,

clk_core_init_rate_req(core, &req, req_rate);

+ clk_core_request_dump(core, &req);
+
ret = clk_core_round_rate_nolock(core, &req);

+ clk_core_request_dump(core, &req);
+
/* restore the protection */
clk_core_rate_restore_protect(core, cnt);

Alternatively, can I easily get a MT8195 device to test more easily?

Thanks!
Maxime

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