Date:   Wed, 12 Oct 2022 14:14:39 +0200
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     sboyd@...nel.org, mturquette@...libre.com, matthias.bgg@...il.com,
        chun-jie.chen@...iatek.com, miles.chen@...iatek.com,
        wenst@...omium.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] clk: mediatek: clk-mux: Add .determine_rate() callback

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).

> 
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index c3c3f8c07258..ffbee00bd7cf 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -1545,6 +1545,8 @@ static int clk_core_round_rate_nolock(struct clk_core *core,
>   	if (core->flags & CLK_SET_RATE_PARENT) {
>   		struct clk_rate_request parent_req;
> 
> +		memset(&parent_req, 0, sizeof(parent_req));
> +
>   		clk_core_forward_rate_req(core, req, core->parent, &parent_req, req->rate);
>   		ret = clk_core_round_rate_nolock(core->parent, &parent_req);
>   		if (ret)
> 
> Thanks!
> Maxime

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