Synchronization is recommended before disabling the trace registers to prevent any start or stop points being speculative at the point of disabling the unit (section 7.3.77 of ARM IHI 0064D).

Synchronization is also recommended after programming the trace registers to ensure all updates are committed prior to normal code resuming (section 4.3.7 of ARM IHI 0064D).

Let's ensure these syncronization points are present in the code and clearly commented.

Note that we could rely on the barriers in CS_LOCK and coresight_disclaim_device_unlocked or the context switch to user space - however coresight may be of use in the kernel.

On armv8 the mb macro is defined as dsb(sy) - Given that the etm4x is only used on armv8 let's directly use dsb(sy) instead of mb(). This removes some ambiguity and makes it easier to correlate the code with the TRM.

Signed-off-by: Andrew Murray andrew.murray@arm.com Reviewed-by: Suzuki K Poulose suzuki.poulose@arm.com CC: stable@vger.kernel.org --- drivers/hwtracing/coresight/coresight-etm4x.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x.c b/drivers/hwtracing/coresight/coresight-etm4x.c index 7ad15651e069..ec9468880c71 100644 --- a/drivers/hwtracing/coresight/coresight-etm4x.c +++ b/drivers/hwtracing/coresight/coresight-etm4x.c @@ -188,6 +188,13 @@ static int etm4_enable_hw(struct etmv4_drvdata *drvdata) dev_err(etm_dev, "timeout while waiting for Idle Trace Status\n");

+ /* + * As recommended by section 4.3.7 ("Synchronization when using the + * memory-mapped interface") of ARM IHI 0064D + */ + dsb(sy); + isb(); + done: CS_LOCK(drvdata->base);

@@ -453,8 +460,12 @@ static void etm4_disable_hw(void *info) /* EN, bit[0] Trace unit enable bit */ control &= ~0x1;

- /* make sure everything completes before disabling */ - mb(); + /* + * Make sure everything completes before disabling, as recommended + * by section 7.3.77 ("TRCVICTLR, ViewInst Main Control Register, + * SSTATUS") of ARM IHI 0064D + */ + dsb(sy); isb(); writel_relaxed(control, drvdata->base + TRCPRGCTLR);