Clock framework deadlock with external SPI clockchip

[Lars-Peter Clausen]

Hi,

I'm currently facing a deadlock in the common clock framework that
unfortunately is not addressed by the reentrancy patches. I have a external
clock chip that is controlled via SPI. So for example to configure the rate
of the clock chip you need to send a SPI message. Naturally the clock
framework will hold the prepare lock while configuring the rate.
Communication in the SPI framework happens asynchronously, spi_sync() will
enqueue a message in the SPI masters queue and then wait using
wait_for_completion(). The master will call complete() once the transfer has
been finished. The SPI master runs in it's own thread in which it processes
the messages. In this thread it also calls clk_set_rate() to configure the
SPI transfer clock rate based on what the message says. Now the deadlock
happens as we try to take the prepare_lock again and since the clock chip
and the SPI master run in different threads the reentrancy code does not
kick in.

The basic sequence is like this:

=== Clock chip driver === === SPI master driver ===
clk_prepare_lock()
spi_sync()
wait_for_completion(X)
clk_get_rate()
clk_prepare_lock() <--- DEADLOCK
clk_prepare_unlock()
...
complete(X)
...
clk_prepare_unlock()

I'm wondering if you have any idea how this can be fixed. In my opinion we'd
need a per clock mutex to address this properly.

Thanks,
- Lars
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[Peter De Schrijver]

One workaround is to leave the SPI masters clock always prepared. A similar
problem can occur with I2C and DVFS using notifiers.

Cheers,

Peter.

[Lars-Peter Clausen]

That's kind of what I'm doing right now as a temporary solution. But we'd
basically need to have all SPI or I2C master drivers to be aware that they
might be used for controlling a external clock chip. And there are also
situations where the workaround does not work. E.g. if the clk API is used
to configure the rate of the SPI SCK signal, since the rate can be set by
the spi message.

- Lars

[Mark Brown]

On Tue, Sep 03, 2013 at 04:22:29PM -0700, Mike Turquette wrote:
> Quoting Lars-Peter Clausen (2013-08-30 06:24:45)

> > === Clock chip driver === === SPI master driver ===
> > clk_prepare_lock()
> > spi_sync()
> > wait_for_completion(X)

> Is there a synchronous equivalent to spi_sync()?

spi_sync() is synchronous? Obviously everything is asynchronous up to
that point but the only way the driver can tell if the I/O completed is
using spi_sync() or the equivalent thereof.

If you're asking if there is a way of doing SPI I/O that isn't multi
threaded then there isn't, in order to keep pushing transfers into the
SPI controller to saturate the bus we have a high priority task to push
more data in. SPI buses can be quite high bandwidth in some
applications.