2018-03-25 14:43:00

by Sinan Kaya

Subject: [PATCH v7 6/7] bnxt_en: Eliminate duplicate barriers on weakly-ordered archs

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.

Create a new wrapper function with relaxed write operator. Use the new
wrapper when a write is following a wmb().

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

Also add mmiowb() so that write code doesn't move outside of scope.

Signed-off-by: Sinan Kaya <[email protected]>
---
drivers/net/ethernet/broadcom/bnxt/bnxt.c | 2 +-
drivers/net/ethernet/broadcom/bnxt/bnxt.h | 9 +++++++++
2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt.c b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
index 1500243..befb538 100644
--- a/drivers/net/ethernet/broadcom/bnxt/bnxt.c
+++ b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
@@ -1922,7 +1922,7 @@ static int bnxt_poll_work(struct bnxt *bp, struct bnxt_napi *bnapi, int budget)
/* Sync BD data before updating doorbell */
wmb();

- bnxt_db_write(bp, db, DB_KEY_TX | prod);
+ bnxt_db_write_relaxed(bp, db, DB_KEY_TX | prod);
}

cpr->cp_raw_cons = raw_cons;
diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt.h b/drivers/net/ethernet/broadcom/bnxt/bnxt.h
index 1989c47..5e453b9 100644
--- a/drivers/net/ethernet/broadcom/bnxt/bnxt.h
+++ b/drivers/net/ethernet/broadcom/bnxt/bnxt.h
@@ -1401,6 +1401,15 @@ static inline u32 bnxt_tx_avail(struct bnxt *bp, struct bnxt_tx_ring_info *txr)
((txr->tx_prod - txr->tx_cons) & bp->tx_ring_mask);
}

+/* For TX and RX ring doorbells with no ordering guarantee*/
+static inline void bnxt_db_write_relaxed(struct bnxt *bp, void __iomem *db,
+ u32 val)
+{
+ writel_relaxed(val, db);
+ if (bp->flags & BNXT_FLAG_DOUBLE_DB)
+ writel_relaxed(val, db);
+}
+
/* For TX and RX ring doorbells */
static inline void bnxt_db_write(struct bnxt *bp, void __iomem *db, u32 val)
{
--
2.7.4

2018-03-25 20:25:32

by Michael Chan

Subject: Re: [PATCH v7 6/7] bnxt_en: Eliminate duplicate barriers on weakly-ordered archs

On Sun, Mar 25, 2018 at 7:39 AM, Sinan Kaya <[email protected]> wrote:
> 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.
>
> Create a new wrapper function with relaxed write operator. Use the new
> wrapper when a write is following a wmb().
>
> Since code already has an explicit barrier call, changing writel() to
> writel_relaxed().
>
> Also add mmiowb() so that write code doesn't move outside of scope.

This line in the patch description is not needed anymore. Other than that,

Acked-by: Michael Chan <[email protected]>

Thanks.
>
> Signed-off-by: Sinan Kaya <[email protected]>