On Fri, Feb 14, 2014 at 3:32 PM, Siva Yerramreddy <siva.krishna.kumar.reddy.yerramreddy@xxxxxxxxx> wrote: > When running dmatest with my yet-to-be submitted driver for the Intel MIC DMA > engine, dmatest detected "dma0chan3-copy5: result #8096161:completion busy > status with src_off=0x0 dst_off=0x0 len=0x40 (0)". This is caused by reading > the used cookie before the completed cookie in dma_cookie_status(), if a DMA > request is submitted in between the two reads, and completes, the completed > cookie will be newer than the used cookie value read previously. Reversing > the order of reads ensures that the completed cookie is for a DMA request > older than the used cookie. > > Reviewed-by: Ashutosh Dixit <ashutosh.dixit@xxxxxxxxx> > Reviewed-by: Sudeep Dutt <sudeep.dutt@xxxxxxxxx> > Reviewed-by: Nikhil Rao <nikhil.rao@xxxxxxxxx> > Signed-off-by: Siva Yerramreddy <siva.krishna.kumar.reddy.yerramreddy@xxxxxxxxx> > --- > drivers/dma/dmaengine.h | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/drivers/dma/dmaengine.h b/drivers/dma/dmaengine.h > index 17f983a..4c96892 100644 > --- a/drivers/dma/dmaengine.h > +++ b/drivers/dma/dmaengine.h > @@ -7,6 +7,7 @@ > > #include <linux/bug.h> > #include <linux/dmaengine.h> > +#include <asm/barrier.h> > > /** > * dma_cookie_init - initialize the cookies for a DMA channel > @@ -69,8 +70,13 @@ static inline enum dma_status dma_cookie_status(struct dma_chan *chan, > { > dma_cookie_t used, complete; > > - used = chan->cookie; > complete = chan->completed_cookie; > + /* > + * If this order is not maintained, used can end up being older than > + * complete > + */ > + smp_rmb(); > + used = chan->cookie; > barrier(); > if (state) { > state->last = complete; Hmm, this patch by itself is problematic because the rmb() needs to be paired with a wmb() otherwise it really has no effect. Is there a wmb() in your driver? I think this does need to be fixed, but we need to provide generic helpers for other drivers. I also think smb_rmb() and smb_wmb() are overkill we should just be able to use smp_store_release() and smp_load_acquire() for this. -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html