Re: Handling clocks on external busses

[Charles Keepax <ckeepax@opensource.wolfsonmicro.com>]

On Tue, Nov 24, 2015 at 05:37:18PM +0000, Charles Keepax wrote:
> Hi,
> 
> When a clock driver is controlling a clock that is controlled
> over I2C / SPI, we need to perform a write on that bus to enable
> the clock. However, such busses often have their own clocks that
> must be enabled. Since all clock prepares are controlled under
> one large mutex this easily causes deadlock. The device is
> waiting for the I2C / SPI write to complete and the I2C / SPI
> driver is waiting for the clock prepare lock to be released so it
> can enable its own clock.
> 
> I have had a bit of a search and it seems the only really advice
> kicking about is that all I2C / SPI drivers should leave the
> clock prepared all the time. Is that intended to be the long term
> solution, should I treat not leaving the clock prepared as a bug?
> 
> Thanks,
> Charles

Adding a few more people for visibility.

So after a bit more digging it seems this has been mitigated slightly
as a lot of SPI driver have been updated to execute transfers in
thread rather than from a worker thread and it seems the clock
framework lets you re-enter the locked sections if called from the
same thread.

I am looking at moving some (in mainline) clocking code into an actual
clocking driver (for a SPI/I2C audio CODEC) but I am rather nervous
about this causing problems for customers with random deadlocks.

I guess the naive solution looks like individual locks per clock, but
from what I have been able to google it looks like there are some
challenges with that approach, does anyone have any links to previous
discussions on that? And any suggestions on how I might approach
this?

Thanks,
Charles