linux-kernel - Re: [PATCH v3 2/3] clk: divider: Switch from .round_rate to .determine

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Date:   Thu, 01 Jul 2021 18:02:46 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Guenter Roeck <linux@...ck-us.net>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     mturquette@...libre.com, linux-clk@...r.kernel.org,
        Neil Armstrong <narmstrong@...libre.com>, jbrunet@...libre.com,
        khilman@...libre.com, linux-kernel@...r.kernel.org,
        linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 2/3] clk: divider: Switch from .round_rate to .determine_rate by default

Quoting Martin Blumenstingl (2021-07-01 13:57:28)
> Hi Guenter,
> 
> On Thu, Jul 1, 2021 at 10:25 PM Guenter Roeck <linux@...ck-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

My guess is that we have drivers copying the clk_ops from the
divider_ops structure and so they are copying over round_rate but not
determine_rate.

> 
> 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...
>