On Tue, Mar 5, 2019 at 5:35 PM dbasehore . <dbasehore@xxxxxxxxxxxx> wrote: > > On Tue, Mar 5, 2019 at 10:49 AM Stephen Boyd <sboyd@xxxxxxxxxx> wrote: > > > > Quoting Derek Basehore (2019-03-04 20:49:31) > > > From: Stephen Boyd <sboyd@xxxxxxxxxxxxxx> > > > > > > Enabling and preparing clocks can be written quite naturally with > > > recursion. We start at some point in the tree and recurse up the > > > tree to find the oldest parent clk that needs to be enabled or > > > prepared. Then we enable/prepare and return to the caller, going > > > back to the clk we started at and enabling/preparing along the > > > way. This also unroll the recursion in unprepare,disable which can > > > just be done in the order of walking up the clk tree. > > > > > > The problem is recursion isn't great for kernel code where we > > > have a limited stack size. Furthermore, we may be calling this > > > code inside clk_set_rate() which also has recursion in it, so > > > we're really not looking good if we encounter a tall clk tree. > > > > > > Let's create a stack instead by looping over the parent chain and > > > collecting clks of interest. Then the enable/prepare becomes as > > > simple as iterating over that list and calling enable. > > > > > > Modified verison of https://lore.kernel.org/patchwork/patch/814369/ > > > -Fixed kernel warning > > > -unrolled recursion in unprepare/disable too > > > > > > Cc: Jerome Brunet <jbrunet@xxxxxxxxxxxx> > > > Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx> > > > Signed-off-by: Derek Basehore <dbasehore@xxxxxxxxxxxx> > > > --- > > > > From the original post: > > > > "I have some vague fear that this may not work if a clk op is framework > > reentrant and attemps to call consumer clk APIs from within the clk ops. > > If the reentrant call tries to add a clk that's already in the list then > > we'll corrupt the list. Ugh." > > > > Do we have this sort of problem here? Or are you certain that we don't > > have clks that prepare or enable something that is already in the > > process of being prepared or enabled? > > I can look into whether anything's doing this and add a WARN_ON which > returns an error if we ever hit that case. If this is happening on > some platform, we'd want to correct that anyways. > Also, if we're ever able to move to another locking scheme (hopefully soon...), we can make the prepare/enable locks non-reentrant. Then if anyone recursively calls back into the framework for another prepare/enable, they will deadlock. I guess that's one way of making sure no one does that. > >
