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SubjectRe: [PATCH] iommu/arm-smmu-v3: Poll for CMDQ drain completion more effectively
On Wed, May 03, 2017 at 04:33:57PM +0100, Robin Murphy wrote:
> On 27/04/17 12:13, sunil.kovvuri@gmail.com wrote:
> > From: Sunil Goutham <sgoutham@cavium.com>
> >
> > Modified polling on CMDQ consumer similar to how polling is done for TLB SYNC
> > completion in SMMUv2 driver. Code changes are done with reference to
> >
> > 8513c8930069 iommu/arm-smmu: Poll for TLB sync completion more effectively
> >
> > Poll timeout has been increased which addresses issue of 100us timeout not
> > sufficient, when command queue is full with TLB invalidation commands.
> >
> > Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
> > Signed-off-by: Geetha <gakula@cavium.com>
> > ---
> > drivers/iommu/arm-smmu-v3.c | 15 ++++++++++++---
> > 1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> > index d412bdd..34599d4 100644
> > --- a/drivers/iommu/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm-smmu-v3.c
> > @@ -379,6 +379,9 @@
> > #define CMDQ_SYNC_0_CS_NONE (0UL << CMDQ_SYNC_0_CS_SHIFT)
> > #define CMDQ_SYNC_0_CS_SEV (2UL << CMDQ_SYNC_0_CS_SHIFT)
> >
> > +#define CMDQ_DRAIN_TIMEOUT_US 1000
> > +#define CMDQ_SPIN_COUNT 10
> > +
> > /* Event queue */
> > #define EVTQ_ENT_DWORDS 4
> > #define EVTQ_MAX_SZ_SHIFT 7
> > @@ -737,7 +740,8 @@ static void queue_inc_prod(struct arm_smmu_queue *q)
> > */
> > static int queue_poll_cons(struct arm_smmu_queue *q, bool drain, bool wfe)
> > {
> > - ktime_t timeout = ktime_add_us(ktime_get(), ARM_SMMU_POLL_TIMEOUT_US);
> > + ktime_t timeout = ktime_add_us(ktime_get(), CMDQ_DRAIN_TIMEOUT_US);
> > + unsigned int spin_cnt, delay = 1;
> >
> > while (queue_sync_cons(q), (drain ? !queue_empty(q) : queue_full(q))) {
> > if (ktime_compare(ktime_get(), timeout) > 0)
> > @@ -746,8 +750,13 @@ static int queue_poll_cons(struct arm_smmu_queue *q, bool drain, bool wfe)
> > if (wfe) {
> > wfe();
> > } else {
> > - cpu_relax();
> > - udelay(1);
> > + for (spin_cnt = 0;
> > + spin_cnt < CMDQ_SPIN_COUNT; spin_cnt++) {
> > + cpu_relax();
> > + continue;
> > + }
> > + udelay(delay);
> > + delay *= 2;
>
> Sorry, I can't make sense of this. The referenced commit uses the spin
> loop to poll opportunistically a few times before delaying. This loop
> just adds a short open-coded udelay to an exponential udelay, and it's
> not really clear that that's any better than a fixed udelay (especially
> as the two cases in which we poll are somewhat different).
>
> What's wrong with simply increasing the timeout value alone?

I asked that the timeout is only increased for the drain case, and that
we fix the issue here where we udelat if cons didn't move immediately:

http://lists.infradead.org/pipermail/linux-arm-kernel/2017-April/503389.html

but I don't think the patch above actually achieves any of that.

Will

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