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Subject: Re: [PATCH v5 00/14] Add BLK_CTL support for i.MX8MP

On Wed, Mar 3, 2021 at 4:54 AM Marek Vasut <[email protected]> wrote:
>
> On 3/3/21 11:47 AM, Abel Vesa wrote:
> > On 20-11-03 13:18:12, Abel Vesa wrote:
> >> The BLK_CTL according to HW design is basically the wrapper of the entire
> >> function specific group of IPs and holds GPRs that usually cannot be placed
> >> into one specific IP from that group. Some of these GPRs are used to control
> >> some clocks, other some resets, others some very specific function that does
> >> not fit into clocks or resets. Since the clocks are registered using the i.MX
> >> clock subsystem API, the driver is placed into the clock subsystem, but it
> >> also registers the resets. For the other functionalities that other GPRs might
> >> have, the syscon is used.
> >>
> >
> > This approach seems to be introducing a possible ABBA deadlock due to
> > the core clock and genpd locking. Here is a backtrace I got from Pete
> > Zhang (he reported the issue on the internal mailing list):
> >
> > [ 11.667711][ T108] -> #1 (&genpd->mlock){+.+.}-{3:3}:
> > [ 11.675041][ T108] __lock_acquire+0xae4/0xef8
> > [ 11.680093][ T108] lock_acquire+0xfc/0x2f8
> > [ 11.684888][ T108] __mutex_lock+0x90/0x870
> > [ 11.689685][ T108] mutex_lock_nested+0x44/0x50
> > [ 11.694826][ T108] genpd_lock_mtx+0x18/0x24
> > [ 11.699706][ T108] genpd_runtime_resume+0x90/0x214 (hold genpd->mlock)
> > [ 11.705194][ T108] __rpm_callback+0x80/0x2c0
> > [ 11.710160][ T108] rpm_resume+0x468/0x650
> > [ 11.714866][ T108] __pm_runtime_resume+0x60/0x88
> > [ 11.720180][ T108] clk_pm_runtime_get+0x28/0x9c
> > [ 11.725410][ T108] clk_disable_unused_subtree+0x8c/0x144
> > [ 11.731420][ T108] clk_disable_unused_subtree+0x124/0x144
> > [ 11.737518][ T108] clk_disable_unused+0xa4/0x11c (hold prepare_lock)
> > [ 11.742833][ T108] do_one_initcall+0x98/0x178
> > [ 11.747888][ T108] do_initcall_level+0x9c/0xb8
> > [ 11.753028][ T108] do_initcalls+0x54/0x94
> > [ 11.757736][ T108] do_basic_setup+0x24/0x30
> > [ 11.762614][ T108] kernel_init_freeable+0x70/0xa4
> > [ 11.768014][ T108] kernel_init+0x14/0x18c
> > [ 11.772722][ T108] ret_from_fork+0x10/0x18
> >
> > [ 11.777512][ T108] -> #0 (prepare_lock){+.+.}-{3:3}:
> > [ 11.784749][ T108] check_noncircular+0x134/0x13c
> > [ 11.790064][ T108] validate_chain+0x590/0x2a04
> > [ 11.795204][ T108] __lock_acquire+0xae4/0xef8
> > [ 11.800258][ T108] lock_acquire+0xfc/0x2f8
> > [ 11.805050][ T108] __mutex_lock+0x90/0x870
> > [ 11.809841][ T108] mutex_lock_nested+0x44/0x50
> > [ 11.814983][ T108] clk_unprepare+0x5c/0x100 ((hold prepare_lock))
> > [ 11.819864][ T108] imx8m_pd_power_off+0xac/0x110
> > [ 11.825179][ T108] genpd_power_off+0x1b4/0x2dc
> > [ 11.830318][ T108] genpd_power_off_work_fn+0x38/0x58 (hold genpd->mlock)
> > [ 11.835981][ T108] process_one_work+0x270/0x444
> > [ 11.841208][ T108] worker_thread+0x280/0x4e4
> > [ 11.846176][ T108] kthread+0x13c/0x14
> > [ 11.850621][ T108] ret_from_fork+0x10/0x18
> >
> > Now, this has been reproduced only on the NXP internal tree, but I think
> > it is pretty obvious this could happen in upstream too, with this
> > patchset applied.
> >
> > First, my thought was to change the prepare_lock/enable_lock in clock
> > core, from a global approach to a per clock basis. But that doesn't
> > actually fix the issue.
> >
> > The usecase seen above is due to clk_disable_unused, but the same could
> > happen when a clock consumer calls prepare/unprepare on a clock.
> >
> > I guess the conclusion is that the current state of the clock core and
> > genpd implementation does not support a usecase where a clock controller
> > has a PD which in turn uses another clock (from another clock controller).
> >
> > Jacky, Pete, did I miss anything here ?
>
> Just for completeness, I have a feeling I already managed to trigger
> this and discussed this with Lucas before, so this concern is certainly
> valid.

I know it may not be ideal, someone tied a soft-reset and soft-enable
to the driver of the Hantro VPU on the IMXMQ [1], and I wonder if
something similar could be done for the drivers who are consumers of
the clocks.

For example:

lcdif could request the power domain.
The power domain soft-resets and enables bus clock (vis syscon)
After successful enabling of power-domain, the LCDIF requests the soft
reset and respective clock bits (also via syscon) similar to how [1]
and [2] do it for the Hantro VPU.

The syscon node could be a common node similar to what was done in
[2], and multiple consumers could control when each soft-reset and
clock-enable get activated. I know it's probably more of an abuse of
the syscon system, but having the individual drivers control the order
of operations might be safer than trying to create a one-size-fits-all
driver.

adam
[1] - https://patchwork.kernel.org/project/linux-arm-kernel/patch/[email protected]/
[2] - https://patchwork.kernel.org/project/linux-arm-kernel/patch/[email protected]/