Clock framework deadlock with external SPI clockchip

[Peter De Schrijver]

On Fri, Aug 30, 2013 at 03:24:45PM +0200, Lars-Peter Clausen wrote:
> 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.

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

Cheers,

Peter.