Avoid a long delay when a busy bit is still set and has to be polled
again.
Measurements on a system with 2 Opals (6097F) and one Agate (6352)
show that even with this much tighter loop, we have about a 50% chance
of the bit being cleared on the first poll, all other accesses see the
bit being cleared on the second poll.
On a standard MDIO bus running MDC at 2.5MHz, a single access with 32
bits of preamble plus 32 bits of data takes 64*(1/2.5MHz) = 25.6us.
This means that mv88e6xxx_smi_direct_wait took 26us + CPU overhead in
the fast scenario, but 26us + 1500us + 26us + CPU overhead in the slow
case - bringing the average close to 1ms.
With this change in place, the slow case is closer to 2*26us + CPU
overhead, with the average well below 100us - a 10x improvement.
This translates to real-world winnings. On a 3-chip 20-port system,
the modprobe time drops by 88%:
Before:
root@coronet:~# time modprobe mv88e6xxx
real 0m 15.99s
user 0m 0.00s
sys 0m 1.52s
After:
root@coronet:~# time modprobe mv88e6xxx
real 0m 2.21s
user 0m 0.00s
sys 0m 1.54s
Signed-off-by: Tobias Waldekranz <[email protected]>
---
drivers/net/dsa/mv88e6xxx/chip.c | 10 +++++++---
drivers/net/dsa/mv88e6xxx/smi.c | 8 ++++++--
2 files changed, 13 insertions(+), 5 deletions(-)
diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
index 58ca684d73f7..de8a568a8c53 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.c
+++ b/drivers/net/dsa/mv88e6xxx/chip.c
@@ -86,12 +86,16 @@ int mv88e6xxx_write(struct mv88e6xxx_chip *chip, int addr, int reg, u16 val)
int mv88e6xxx_wait_mask(struct mv88e6xxx_chip *chip, int addr, int reg,
u16 mask, u16 val)
{
+ const unsigned long timeout = jiffies + msecs_to_jiffies(50);
u16 data;
int err;
int i;
- /* There's no bus specific operation to wait for a mask */
- for (i = 0; i < 16; i++) {
+ /* There's no bus specific operation to wait for a mask. Even
+ * if the initial poll takes longer than 50ms, always do at
+ * least one more attempt.
+ */
+ for (i = 0; time_before(jiffies, timeout) || (i < 2); i++) {
err = mv88e6xxx_read(chip, addr, reg, &data);
if (err)
return err;
@@ -99,7 +103,7 @@ int mv88e6xxx_wait_mask(struct mv88e6xxx_chip *chip, int addr, int reg,
if ((data & mask) == val)
return 0;
- usleep_range(1000, 2000);
+ cpu_relax();
}
dev_err(chip->dev, "Timeout while waiting for switch\n");
diff --git a/drivers/net/dsa/mv88e6xxx/smi.c b/drivers/net/dsa/mv88e6xxx/smi.c
index 282fe08db050..466d2aaa9fcb 100644
--- a/drivers/net/dsa/mv88e6xxx/smi.c
+++ b/drivers/net/dsa/mv88e6xxx/smi.c
@@ -55,11 +55,15 @@ static int mv88e6xxx_smi_direct_write(struct mv88e6xxx_chip *chip,
static int mv88e6xxx_smi_direct_wait(struct mv88e6xxx_chip *chip,
int dev, int reg, int bit, int val)
{
+ const unsigned long timeout = jiffies + msecs_to_jiffies(50);
u16 data;
int err;
int i;
- for (i = 0; i < 16; i++) {
+ /* Even if the initial poll takes longer than 50ms, always do
+ * at least one more attempt.
+ */
+ for (i = 0; time_before(jiffies, timeout) || (i < 2); i++) {
err = mv88e6xxx_smi_direct_read(chip, dev, reg, &data);
if (err)
return err;
@@ -67,7 +71,7 @@ static int mv88e6xxx_smi_direct_wait(struct mv88e6xxx_chip *chip,
if (!!(data & BIT(bit)) == !!val)
return 0;
- usleep_range(1000, 2000);
+ cpu_relax();
}
return -ETIMEDOUT;
--
2.25.1
On Fri, Jan 28, 2022 at 11:49:37AM +0100, Tobias Waldekranz wrote:
> Avoid a long delay when a busy bit is still set and has to be polled
> again.
>
> Measurements on a system with 2 Opals (6097F) and one Agate (6352)
> show that even with this much tighter loop, we have about a 50% chance
> of the bit being cleared on the first poll, all other accesses see the
> bit being cleared on the second poll.
>
> On a standard MDIO bus running MDC at 2.5MHz, a single access with 32
> bits of preamble plus 32 bits of data takes 64*(1/2.5MHz) = 25.6us.
>
> This means that mv88e6xxx_smi_direct_wait took 26us + CPU overhead in
> the fast scenario, but 26us + 1500us + 26us + CPU overhead in the slow
> case - bringing the average close to 1ms.
>
> With this change in place, the slow case is closer to 2*26us + CPU
> overhead, with the average well below 100us - a 10x improvement.
>
> This translates to real-world winnings. On a 3-chip 20-port system,
> the modprobe time drops by 88%:
>
> Before:
>
> root@coronet:~# time modprobe mv88e6xxx
> real 0m 15.99s
> user 0m 0.00s
> sys 0m 1.52s
>
> After:
>
> root@coronet:~# time modprobe mv88e6xxx
> real 0m 2.21s
> user 0m 0.00s
> sys 0m 1.54s
>
> Signed-off-by: Tobias Waldekranz <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Andrew