Hi Sylwester, On Tuesday 15 October 2013 22:04:17 Sylwester Nawrocki wrote: > Hi, > > (adding linux-media mailing list at Cc) > > On 09/25/2013 11:47 AM, Laurent Pinchart wrote: > > On Tuesday 24 September 2013 23:38:44 Sylwester Nawrocki wrote: > [...] > > >> The only issue I found might be at the omap3isp driver, which provides > >> clock to its sub-drivers and takes reference on the sub-driver modules. > >> When sub-driver calls clk_get() all modules would get locked in memory, > >> due to circular reference. One solution to that could be to pass NULL > >> struct device pointer, as in the below patch. > > > > Doesn't that introduce race conditions ? If the sub-drivers require the > > clock, they want to be sure that the clock won't disappear beyond their > > backs. I agree that the circular dependency needs to be solved somehow, > > but we probably need a more generic solution. The problem will become > > more widespread in the future with DT-based device instantiation in both > > V4L2 and KMS. > > I'm wondering whether subsystems and drivers itself should be written so > they deal with such dependencies they are aware of. > > There is similar situation in the regulator API, regulator_get() simply > takes a reference on a module providing the regulator object. > > Before a "more generic solution" is available, what do you think about > keeping obtaining a reference on a clock provider module in clk_get() and > doing clk_get(), clk_prepare_enable(), ..., clk_unprepare_disable(), > clk_put() in sub-driver whenever a clock is actively used, to avoid > permanent circular reference ? That's a workaround I can live with in the short term, as long as we work on a generic solution to this problem. It will bite us back in the not too distant future if we pretend to forget about it. -- Regards, Laurent Pinchart
