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    SubjectRe: Locking in the clk API
    On 01/15/2011 03:53 PM, Russell King - ARM Linux wrote:
    > On Sat, Jan 15, 2011 at 03:02:25PM +0100, Christer Weinigel wrote:
    >> On platforms that need to sleep to enable the UART clock, configuring
    >> the UART as the kernel console should be equivalent to userspace opening
    >> the UART device, i.e. enable the clock. At least to me that feels like
    >> an acceptable tradeoff, and if I wanted to save the last bit of power
    >> I'll have to refrain from using UART as the kernel console.
    >
    > Well, we're not discussing a _new_ API here - we're discussing an API
    > with existing users which works completely fine on the devices its
    > used, with differing expectations between implementations.

    Yes, so to fulfil the requirement that printk needs to call clk_enable
    from atomic contexts, document that clk_enable can not sleep. Or add
    the clk_enable_atomic call and modify printk to use it.

    >> Both of these feel like they should use a call such as clk_get_atomic
    >> and be able to handle EWOULDBLOCK/EAGAIN (or whatever error code is used
    >> to indicate that it would have to sleep) and delegate to a worker thread
    >> to enable the clock. To catch uses of plain clk_enable from atomic
    >> contects, add a WARN_ON/BUG_ON(in_atomic()). It won't catch everything,
    >> but would help a bit at least.
    >
    > We've never allowed clk_get() to be called in interruptible context,
    > so that's not the issue. The issue is purely about clk_enable() and
    > clk_disable() and whether they should be able to be called in atomic
    > context or not.

    My bad, it should have said "clk_enable_atomic".

    > There's been a lot of talk on this issue for ages with no real progress
    > that I'm just going to repeat: let's unify those implementations which
    > use a spinlock for their clks into one consolidated solution, and
    > a separate consolidated solution for those which use a mutex.
    >
    > This will at least allow us to have _some_ consolidation of the existing
    > implementations - and it doesn't add anything to the problem at hand.
    > It might actually help identify what can be done at code level to resolve
    > this issue.

    Won't that cause a lot of code duplication? If it's possible to have
    one sane implementation, why not go for it at once?

    /Christer


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