Re: [PATCH v3 2/3] clk: divider: Switch from .round_rate to .determine_rate by default

From: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
To: Guenter Roeck <linux@roeck-us.net>
Cc: mturquette@baylibre.com, sboyd@kernel.org,
	linux-clk@vger.kernel.org,
	Neil Armstrong <narmstrong@baylibre.com>,
	jbrunet@baylibre.com, khilman@baylibre.com,
	linux-kernel@vger.kernel.org, linux-amlogic@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 2/3] clk: divider: Switch from .round_rate to .determine_rate by default
Date: Thu, 1 Jul 2021 22:57:28 +0200	[thread overview]
Message-ID: <CAFBinCC2KB-_pOenpWPknCuHV+CCjhP5hqukSkwD3qwRe6OtQw@mail.gmail.com> (raw)
In-Reply-To: <20210701202540.GA1085600@roeck-us.net>

Hi Guenter,

On Thu, Jul 1, 2021 at 10:25 PM Guenter Roeck <linux@roeck-us.net> wrote:
[...]
> [    0.000000] [<c07be330>] (clk_core_determine_round_nolock) from [<c07c5480>] (clk_core_set_rate_nolock+0x184/0x294)
> [    0.000000] [<c07c5480>] (clk_core_set_rate_nolock) from [<c07c55c0>] (clk_set_rate+0x30/0x64)
> [    0.000000] [<c07c55c0>] (clk_set_rate) from [<c163c310>] (imx6ul_clocks_init+0x2798/0x2a44)
> [    0.000000] [<c163c310>] (imx6ul_clocks_init) from [<c162a4e4>] (of_clk_init+0x180/0x26c)
> [    0.000000] [<c162a4e4>] (of_clk_init) from [<c1604d34>] (time_init+0x20/0x30)
> [    0.000000] [<c1604d34>] (time_init) from [<c1600e0c>] (start_kernel+0x4c8/0x6cc)
> [    0.000000] [<c1600e0c>] (start_kernel) from [<00000000>] (0x0)
> [    0.000000] Code: bad PC value
> [    0.000000] ---[ end trace 7009a0f298fd39e9 ]---
> [    0.000000] Kernel panic - not syncing: Attempted to kill the idle task!
>
> Bisct points to this patch as culprit. Reverting it fixes the problem.
sorry for breaking imx6 - and at the same time: thanks for reporting this!

Do you have some additional information about this crash (which clock
this relates to, file and line number, etc.)?
I am struggling to understand the cause of this NULL dereference
My patch doesn't change the clk_core_determine_round_nolock()
implementation and the new determine_rate code-path (inside that
function) doesn't seem to be more fragile in terms of NULL values
compared to the round_rate code-path.
Instead I think it's more likely that the problem is somewhere within
clk_divider_determine_rate() (or in any helper function it uses), but
that doesn't show up in the trace

I don't have any imx6 board myself and so far I am unable to reproduce
this crash on any hardware I have.
However, if it's a problem in my clk-divider.c changes then I'd like
to find the cause (ASAP) because possibly more SoCs may be broken...

Best regards,
Martin