linux-kernel - [PATCH v2] net: ocelot: fix wrong time

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Date:   Sat, 21 May 2022 00:31:15 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     vladimir.oltean@....com, claudiu.manoil@....com,
        alexandre.belloni@...tlin.com, UNGLinuxDriver@...rochip.com,
        kuba@...nel.org, clement.leger@...tlin.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pavel Skripkin <paskripkin@...il.com>,
        Andrew Lunn <andrew@...n.ch>
Subject: [PATCH v2] net: ocelot: fix wrong time_after usage

Accidentally noticed, that this driver is the only user of
while (time_after(jiffies...)).

It looks like typo, because likely this while loop will finish after 1st
iteration, because time_after() returns true when 1st argument _is after_
2nd one.

There is one possible problem with this poll loop: the scheduler could put
the thread to sleep, and it does not get woken up for
OCELOT_FDMA_CH_SAFE_TIMEOUT_US. During that time, the hardware has done
its thing, but you exit the while loop and return -ETIMEDOUT.

Fix it by using sane poll API that avoids all problems described above

Fixes: 753a026cfec1 ("net: ocelot: add FDMA support")
Suggested-by: Andrew Lunn <andrew@...n.ch>
Signed-off-by: Pavel Skripkin <paskripkin@...il.com>
---

I can't say if 0 is a good choise for 5th readx_poll_timeout() argument,
so this patch is build-tested only.

Testing and suggestions are welcomed!

Changes since v1:
	- Fixed typos in title and commit message
	- Remove while loop and use readx_poll_timeout as suggested by
	  Andrew

---
 drivers/net/ethernet/mscc/ocelot_fdma.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/mscc/ocelot_fdma.c b/drivers/net/ethernet/mscc/ocelot_fdma.c
index dffa597bffe6..82abfa8394c8 100644
--- a/drivers/net/ethernet/mscc/ocelot_fdma.c
+++ b/drivers/net/ethernet/mscc/ocelot_fdma.c
@@ -94,19 +94,18 @@ static void ocelot_fdma_activate_chan(struct ocelot *ocelot, dma_addr_t dma,
 	ocelot_fdma_writel(ocelot, MSCC_FDMA_CH_ACTIVATE, BIT(chan));
 }
 
+static u32 ocelot_fdma_read_ch_safe(struct ocelot *ocelot)
+{
+	return ocelot_fdma_readl(ocelot, MSCC_FDMA_CH_SAFE);
+}
+
 static int ocelot_fdma_wait_chan_safe(struct ocelot *ocelot, int chan)
 {
-	unsigned long timeout;
 	u32 safe;
 
-	timeout = jiffies + usecs_to_jiffies(OCELOT_FDMA_CH_SAFE_TIMEOUT_US);
-	do {
-		safe = ocelot_fdma_readl(ocelot, MSCC_FDMA_CH_SAFE);
-		if (safe & BIT(chan))
-			return 0;
-	} while (time_after(jiffies, timeout));
-
-	return -ETIMEDOUT;
+	return readx_poll_timeout(ocelot_fdma_read_ch_safe, ocelot, safe,
+				  safe & BIT(chan), 0,
+				  OCELOT_FDMA_CH_SAFE_TIMEOUT_US);
 }
 
 static void ocelot_fdma_dcb_set_data(struct ocelot_fdma_dcb *dcb,
-- 
2.36.1