On Tue, Mar 20, 2018 at 10:23:16AM -0500, Sinan Kaya wrote:
> On 3/20/2018 9:54 AM, Jason Gunthorpe wrote:
> > On Mon, Mar 19, 2018 at 10:47:47PM -0400, Sinan Kaya wrote:
> >> Code includes barrier() followed by writel(). writel() already has a
> >> barrier on some architectures like arm64.
> >>
> >> This ends up CPU observing two barriers back to back before executing the
> >> register write.
> >>
> >> Create a new wrapper function with relaxed write operator. Use the new
> >> wrapper when a write is following a barrier().
> >>
> >> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
> >> drivers/infiniband/hw/nes/nes.h | 5 +++++
> >> drivers/infiniband/hw/nes/nes_hw.c | 21 ++++++++++++++-------
> >> drivers/infiniband/hw/nes/nes_mgt.c | 15 ++++++++++-----
> >> drivers/infiniband/hw/nes/nes_nic.c | 2 +-
> >> drivers/infiniband/hw/nes/nes_utils.c | 3 ++-
> >> drivers/infiniband/hw/nes/nes_verbs.c | 5 +++--
> >> 6 files changed, 35 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/drivers/infiniband/hw/nes/nes.h b/drivers/infiniband/hw/nes/nes.h
> >> index 00c27291..85e007d 100644
> >> +++ b/drivers/infiniband/hw/nes/nes.h
> >> @@ -387,6 +387,11 @@ static inline void nes_write_indexed(struct nes_device *nesdev, u32 reg_index, u
> >> spin_unlock_irqrestore(&nesdev->indexed_regs_lock, flags);
> >> }
> >>
> >> +static inline void nes_write32_relaxed(void __iomem *addr, u32 val)
> >> +{
> >> + writel_relaxed(val, addr);
> >> +}
> >
> > This wrapper is pointless, let us not add more..
> >
> >> static inline void nes_write32(void __iomem *addr, u32 val)
> >> {
> >> writel(val, addr);
> >> diff --git a/drivers/infiniband/hw/nes/nes_hw.c b/drivers/infiniband/hw/nes/nes_hw.c
> >> index 18a7de1..568e17d 100644
> >> +++ b/drivers/infiniband/hw/nes/nes_hw.c
> >> @@ -1257,7 +1257,8 @@ int nes_destroy_cqp(struct nes_device *nesdev)
> >>
> >> barrier();
> >> /* Ring doorbell (5 WQEs) */
> >> - nes_write32(nesdev->regs+NES_WQE_ALLOC, 0x05800000 | nesdev->cqp.qp_id);
> >> + nes_write32_relaxed(nesdev->regs+NES_WQE_ALLOC,
> >> + 0x05800000 | nesdev->cqp.qp_id);
> >
> > barrier() is not strong enough to order writel, so this doesn't seem
> > right?
> >
> > It is probably noteven strong enough for what this driver thinks it is
> > doing.. This driver is essentially dead and broken, probably just
> > don't change it.
>
> Just for the sake of other changes in netdev directory and my education...
>
> barrier() on ARM is a wmb(). It should be a compiler barrier on intel.
>
> You are saying barrier() should have been a wmb(), right?
Yes, that is my understanding.. barrier() is supposed to be a very
weak barrier that just ensures the CPU is locally consistent with
itself. It doesn't say anything about DMA access, or SMP cases.
I don't think it is supposed to order anything related to
writel_relaxed()
> I have gone through similar exercise on netdev directory and changed
>
> barrier()
> writel()
>
> to
>
> barrier()
> writel_relaxed()
>
> Do you see any problem with this?
Seems dangerous as a mechanical change to me, it really depends on why
the driver author put barrier() there.
In this case, I strongly suspect nes really intended to say wmb()
Jason
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