[PATCH v4 1/6] RDMA/bnxt_re: Eliminate duplicate barriers on weakly-ordered archs

On Tue, Mar 20, 2018 at 10:00:49AM -0500, Sinan Kaya wrote:

On 3/20/2018 9:48 AM, Jason Gunthorpe wrote:

quoted

On Mon, Mar 19, 2018 at 10:47:43PM -0400, Sinan Kaya wrote:

quoted

Code includes wmb() 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.

Since code already has an explicit barrier call, changing writel() to
writel_relaxed().

Signed-off-by: Sinan Kaya <redacted>
 drivers/infiniband/hw/bnxt_re/qplib_rcfw.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c b/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c
index 8329ec6..4a6b981 100644
+++ b/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c

@@ -181,10 +181,10 @@ static int __send_message(struct bnxt_qplib_rcfw *rcfw, struct cmdq_base *req,
 
 	/* ring CMDQ DB */
 	wmb();
-	writel(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
-	       rcfw->cmdq_bar_reg_prod_off);
-	writel(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
-	       rcfw->cmdq_bar_reg_trig_off);
+	writel_relaxed(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
+		       rcfw->cmdq_bar_reg_prod_off);
+	writel_relaxed(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
+		       rcfw->cmdq_bar_reg_trig_off);

Woah, this may not be safe..

The definition of writel_relaxed() is that it is fully unordered, so
the above two writes may change order now. Broadcom guys would have to
ack if that it is OK or not for their hardware.

In general this is not an OK approach for a mechanical
conversion.. Only the first writel can be convereted.

You need to check all your patches to make sure there are no
subsequent writel's in the places touched.

I paid special attention to this one and went to check the barriers
document. According to the document, writes (whether it is relaxed or not)
are always observed by the HW inorder with respect to each other.

Oh interesting, that document got revised to make writel_relaxed less
relaxed a few years ago, didn't know that. Thanks.

However, this is still not OK, the full code is:

        /* ring CMDQ DB */
        wmb();
        writel(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
               rcfw->cmdq_bar_reg_prod_off);
        writel(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
               rcfw->cmdq_bar_reg_trig_off);
done:
        spin_unlock_irqrestore(&cmdq->lock, flags);


And the definition of _relaxed allows the writes to order outside the
spinlock region, which is very likely to be wrong in this driver.

I'm not sure adding a mmiowb() just to use a writel_relaxed is any
sort of win though?

Jason