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Subject: Re: [PATCH] clk: meson-gxbb: set fclk_div3 as CLK_IS_CRITICAL
From:   Christian Hewitt <christianshewitt@gmail.com>
In-Reply-To: <ed6a889eeec4e2809f0840c9e8188b5e4b24fa76.camel@baylibre.com>
Date:   Tue, 16 Oct 2018 08:54:05 +0400
Cc:     Michael Turquette <mturquette@baylibre.com>,
        Neil Armstrong <narmstrong@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Carlo Caione <carlo@caione.org>,
        Kevin Hilman <khilman@baylibre.com>,
        linux-amlogic@lists.infradead.org, linux-clk@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
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Message-Id: <8A3A1316-DC7F-4C5A-9DD8-C115EBB613BB@gmail.com>
References: <1539425086-10997-1-git-send-email-christianshewitt@gmail.com>
 <20181013160810.88481.14237@resonance>
 <ed6a889eeec4e2809f0840c9e8188b5e4b24fa76.camel@baylibre.com>
To:     Jerome Brunet <jbrunet@baylibre.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 15 Oct 2018, at 9:07 pm, Jerome Brunet <jbrunet@baylibre.com> wrote:
>=20
> On Sat, 2018-10-13 at 18:08 +0200, Michael Turquette wrote:
>> Quoting Christian Hewitt (2018-10-13 12:04:46)
>>> On the Khadas VIM2 (GXM) and LePotato (GXL) board there are problems
>>> with reboot; e.g. a ~60 second delay between issuing reboot and the
>>> board power cycling (and in some OS configurations reboot will fail
>>> and require manual power cycling).
>>>=20
>>> Similar to 'commit c987ac6f1f088663b6dad39281071aeb31d450a8 ("clk:
>>> meson-gxbb: set fclk_div2 as CLK_IS_CRITICAL")' the SCPI Cortex-M4
>>> Co-Processor seems to depend on FCLK_DIV3 being operational.
>>>=20
>>> Bisect gives 'commit 05f814402d6174369b3b29832cbb5eb5ed287059 ("clk:
>>> meson: add fdiv clock gates") between 4.16 and 4.16-rc1 as the first
>>> bad commit. This added support for the missing clock gates before =
the
>>> fixed PLL fixed dividers (FCLK_DIVx) and the clock framework which
>>> disabled all the unused fixed dividers, thus it disabled a critical
>>> clock path for the SCPI Co-Processor.
>>>=20
>>> This change simply sets the FCLK_DIV3 gate as critical to ensure
>>> nothing can disable it.
>>=20
>> I'm a bit skeptical of this. If FCLK_DIV3 is gated at run-time, there =
is
>> no side effect other than long and/or failed reboot?
>>=20
>> Seems like someone should be managing this clock, and simply leaving =
it
>> on all the time isn't necessarily the right approach. Any chance that
>> you can dig into this behavior to better understand it?
>>=20
>> It's easy to solve issues by leaving clocks on all the time, but this
>> becomes a problem later on when trying to tune a device for power.
>=20
> Hi Mike,
>=20
> I totally agree with you and, in perfect a world, I would prefer not =
to use this
> CLK_IS_CRITICAL at all. It looks like a cheap fix for:
>=20
> "this is required, I don't for what but it is, so please leave it on"
>=20
> The problem is this issue is a regression. We added a few gates to =
better model
> the clock tree a while ago. Before, those those clock were left =
enabled by the
> bootloader.
>=20
> Now that linux turn them off, we are learning a few things about what =
the FWs
> are doing behind our backs.
>=20
> Among the 5 clocks which got a new gates, only 2 needs to back the old =
way using
> CLK_IS_CRITICAL, so this is still an improvement.
>=20
>>=20
>> Also, if this commit really is the right fix, it should include a
>> comment for FCLK_DIV3 stating why the CLK_IS_CRITICAL flag was set,
>> which may be helpful later on to know if it is safe to remove it. =
Same
>> is true for FCLK_DIV2 in c987ac6f1f088663b6dad39281071aeb31d450a8.
>=20
> +1 There should be FIXME notice in the driver explaining why we put =
that flag in
> the first place, so we can remove it as soon as a driver properly =
handle this
> clock.
>=20
> Christian, is Ok if I amend your patch, or do you prefer to post a v2 =
?
> Mike, with explained, is this change OK with you ?

I=E2=80=99m happy for you to amend, it=E2=80=99s easier for me.
Christian

>>=20
>> Regards,
>> Mike
>>=20
>>>=20
>>> Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
>>> ---
>>> drivers/clk/meson/gxbb.c | 1 +
>>> 1 file changed, 1 insertion(+)
>>>=20
>>> diff --git a/drivers/clk/meson/gxbb.c b/drivers/clk/meson/gxbb.c
>>> index 86d3ae5..4c8925d 100644
>>> --- a/drivers/clk/meson/gxbb.c
>>> +++ b/drivers/clk/meson/gxbb.c
>>> @@ -509,6 +509,7 @@ static struct clk_fixed_factor =
gxbb_fclk_div3_div =3D {
>>>                .ops =3D &clk_fixed_factor_ops,
>>>                .parent_names =3D (const char *[]){ "fixed_pll" },
>>>                .num_parents =3D 1,
>>> +               .flags =3D CLK_IS_CRITICAL,
>>>        },
>>> };
>>>=20
>>> --=20
>>> 2.7.4