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

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Date:   Mon, 31 Dec 2018 23:56:40 +0100
From:   Andreas Färber <afaerber@...e.de>
To:     Mark Brown <broonie@...nel.org>
Cc:     Ben Whitten <ben.whitten@...il.com>,
        devicetree <devicetree@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        "linux-lpwan@...ts.infradead.org" <linux-lpwan@...ts.infradead.org>,
        linux-kernel@...r.kernel.org, starnight@...cu.edu.tw,
        linux-clk <linux-clk@...r.kernel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH v3 lora-next 5/5] net: lora: sx125x sx1301: allow radio to
 register as a clk provider

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...]

Regards,
Andreas

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