Re: [PATCH v3 lora-next 5/5] net: lora: sx125x sx1301: allow radio to register as a clk provider

From: Andreas FÃrber
Date: Mon Dec 31 2018 - 17:56:54 EST

Am 31.12.18 um 18:50 schrieb Mark Brown:
> On Sun, Dec 30, 2018 at 11:55:46AM +0100, Andreas Färber wrote:
>> Given that observed symptoms were CPU stalls, workqueue hangs and RCU
>> problems, requiring a power-cycle to recover, I wonder whether we are
>> running into some atomic/locking issue with clk_enable()? Is it valid at
>> all to use SPI/regmap for clk_enable()? If it is, is there a known issue
>> specific to spi-sun6i (A64) in 4.20.0?
>> I already tried setting .disable_locking = true in both regmap_configs.
>> Any suggestions how to further debug?
> You can't use SPI for clk_enable(), clk_enable() needs to be doable in
> atomic context since we need to wait for the bus operations to complete
> (you can start SPI transfers in atomic context but you still need to
> wait for them to complete). Any clocks that are only accessible via a
> slow bus like I2C or SPI need to do the enable/disable in the
> prepare/unprepare operations which aren't done in atomic context.
> regmap can be used in atomic contexts, though you need to configure it
> to use spinlocks instead of mutexes and ensure that no register cache
> allocations happen during I/O (eg, by providing defaults for all
> registers or by not using a cache).

We have .cache_type = REGCACHE_NONE on both bus and spi regmap_configs.

I moved the regmap_field_write() from .enable to .prepare and set
.fast_io = true on both regmap_configs to force using spinlocks, but
same hang as in .enable before...

And same if I set .disable_locking = true on both.

Given that it works with one SPI driver and not with the other,
independent of the locking options applied, I assume my symptoms are not
a regmap-layer issue.

Is it allowed during a .prepare operation to call the mentioned
clk_get_rate(), which ends up calling clk_prepare_lock()?

According to my debug output in spi-sun6i.c our hanging
regmap_field_write() ends up calling sun6i_transfer_one() three times,
the first two look okay, but the third one doesn't make it past the
clk_get_rate() or following if block. [But for now some fireworks...]


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