clk_disable() previously used an ARM barrier, wmb(), to try to ensure that the hardware write completed before continuing. There are some problems with this approach. The first problem is that wmb() only ensures that the write leaves the ARM -- not that it actually reaches the endpoint device. In this case, the endpoint device - either the PRM, CM, or SCM - is three interconnects away from the ARM, and the final interconnect is low-speed. And the OCP interconnects will post the write, who knows how long that will take to complete. So the wmb() is not really what we want. Worse, the wmb() is indiscriminate; it will cause the ARM to flush any other unrelated buffered writes and wait for the local interconnect to acknowledge them - potentially very expensive. This first problem could be fixed by doing a readback of the same PRM/CM/SCM register. Since these devices use a single OCP thread, this will cause the MPU to wait for the write to complete. But the primary problem is a conceptual one: clk_disable() should not need any kind of barrier. clk_enable() needs one since device driver code must not access a device until its clocks are known to be enabled. But clk_disable() has no such restriction. Since blocking the MPU on a PRM/CM/SCM write can be a very high-latency operation - several hundred MPU cycles - it's worth avoiding this barrier if possible. linux-omap source commit is f4aacad2c0ed1055622d5c1e910befece24ef0e2. Signed-off-by: Paul Walmsley <paul@xxxxxxxxx> Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx> --- arch/arm/mach-omap2/clock.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-) diff --git a/arch/arm/mach-omap2/clock.c b/arch/arm/mach-omap2/clock.c index 07b6232..a1ccdcb 100644 --- a/arch/arm/mach-omap2/clock.c +++ b/arch/arm/mach-omap2/clock.c @@ -448,7 +448,7 @@ static void _omap2_clk_disable(struct clk *clk) else v &= ~(1 << clk->enable_bit); _omap2_clk_write_reg(v, clk->enable_reg, clk); - wmb(); + /* No OCP barrier needed here since it is a disable operation */ } void omap2_clk_disable(struct clk *clk) -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
