Re: [PATCH v3 2/2] i2c: aspeed: Acknowledge Tx done with and without ACK irq late

Quan Nguyen
2023-12-11T04:06:35Z

On 08/12/2023 10:31, Quan Nguyen wrote:
> Commit 2be6b47211e1 ("i2c: aspeed: Acknowledge most interrupts early in
> interrupt handler") acknowledges most interrupts early before the slave
> irq handler is executed, except for the "Receive Done Interrupt status"
> which is acknowledged late in the interrupt.
> However, it has been observed that the early acknowledgment of "Transmit
> Done Interrupt Status" (with ACK or NACK) often causes the interrupt to
> be raised in READ REQUEST state, that shows the
> "Unexpected ACK on read request." complaint messages.
>
> Assuming that the "Transmit Done" interrupt should only be acknowledged
> once it is truly processed, this commit fixes that issue by acknowledging
> interrupts for both ACK and NACK cases late in the interrupt handler.
>
> Fixes: 2be6b47211e1 ("i2c: aspeed: Acknowledge most interrupts early in interrupt handler")
> Signed-off-by: Quan Nguyen <quan@os.amperecomputing.com>
> ---
> v3:
> + Fix the unconditinal write when ack the irqs [Andrew]
> + Refactor the code to enhance code readability [Quan]
> + Fix grammar in commit message [Quan]
>
> v2:
> + Split to separate series [Joel]
> + Added the Fixes line [Joel]
> + Fixed multiline comment [Joel]
> + Refactor irq clearing code [Joel, Guenter]
> + Revised commit message [Joel]
> + Revised commit message [Quan]
> + About a note to remind why the readl() should immediately follow the
> writel() to fix the race condition when clearing irq status from commit
> c926c87b8e36 ("i2c: aspeed: Avoid i2c interrupt status clear race
> condition"), I think it looks straight forward in this patch and decided
> not to add that note. [Joel]
>
> v1:
> + First introduced in
> https://lore.kernel.org/all/20210519074934.20712-1-quan@os.amperecomputing.com/
> ---
> drivers/i2c/busses/i2c-aspeed.c | 21 +++++++++++++--------
> 1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-aspeed.c b/drivers/i2c/busses/i2c-aspeed.c
> index 1c2a4f4c4e1b..967a26dd4ffa 100644
> --- a/drivers/i2c/busses/i2c-aspeed.c
> +++ b/drivers/i2c/busses/i2c-aspeed.c
> @@ -617,13 +617,19 @@ static u32 aspeed_i2c_master_irq(struct aspeed_i2c_bus *bus, u32 irq_status)
> static irqreturn_t aspeed_i2c_bus_irq(int irq, void *dev_id)
> {
> struct aspeed_i2c_bus *bus = dev_id;
> - u32 irq_received, irq_remaining, irq_handled;
> + u32 irq_received, irq_remaining, irq_handled, irq_ack_last;
>
> spin_lock(&bus->lock);
> irq_received = readl(bus->base + ASPEED_I2C_INTR_STS_REG);
> - /* Ack all interrupts except for Rx done */
> - writel(irq_received & ~ASPEED_I2CD_INTR_RX_DONE,
> - bus->base + ASPEED_I2C_INTR_STS_REG);
> +
> + /*
> + * Early acking of INTR_RX_DONE and INTR_TX_[ACK|NAK] would indicate HW to
> + * start receiving or sending new data, and this may cause a race condition
> + * as the irq handler has not yet handled these irqs but is being acked.
> + * Let's ack them late at the end of the irq handler when those are truly processed.
> + */
> + irq_ack_last = ASPEED_I2CD_INTR_RX_DONE | ASPEED_I2CD_INTR_TX_ACK | ASPEED_I2CD_INTR_TX_NAK;
> + writel(irq_received & ~irq_ack_last, bus->base + ASPEED_I2C_INTR_STS_REG);

As there will be v4, I'd like to improve this line of code a bit.

During debug, there are significant number of times when the
irq_received has the value of 0x00000004 (INTR_RX_DONE) or 0x00000001
(INTR_TX_ACK). This makes the "irq_received & ~irq_ack_last" turn out to
be 0.

AFAIK, it does not make sense to keep repeatedly writing the 0 to the HW
then try to re-read it with readl() every time. I'd like to change this
HW access conditionally to avoid those unnecessary access.

The change would looks like below:

diff --git a/drivers/i2c/busses/i2c-aspeed.c
b/drivers/i2c/busses/i2c-aspeed.c
index 5511fd46a65e..0f67218cf04a 100644
--- a/drivers/i2c/busses/i2c-aspeed.c
+++ b/drivers/i2c/busses/i2c-aspeed.c
@@ -93,6 +93,10 @@
ASPEED_I2CD_INTR_RX_DONE | \
ASPEED_I2CD_INTR_TX_NAK | \
ASPEED_I2CD_INTR_TX_ACK)
+#define ASPEED_I2CD_INTR_ACK_RX_TX \
+ (ASPEED_I2CD_INTR_RX_DONE | \
+ ASPEED_I2CD_INTR_TX_ACK | \
+ ASPEED_I2CD_INTR_TX_NAK)

/* 0x14 : I2CD Command/Status Register */
#define ASPEED_I2CD_SCL_LINE_STS BIT(18)
@@ -622,10 +626,19 @@ static irqreturn_t aspeed_i2c_bus_irq(int irq,
void *dev_id)

spin_lock(&bus->lock);
irq_received = readl(bus->base + ASPEED_I2C_INTR_STS_REG);
- /* Ack all interrupts except for Rx done */
- writel(irq_received & ~ASPEED_I2CD_INTR_RX_DONE,
- bus->base + ASPEED_I2C_INTR_STS_REG);
- readl(bus->base + ASPEED_I2C_INTR_STS_REG);
+
+ /*
+ * Early acking of INTR_RX_DONE and INTR_TX_[ACK|NAK] would
+ ...
+ */
+ if (irq_received & ~ASPEED_I2CD_INTR_ACK_RX_TX) {
+ writel(irq_received & ~ASPEED_I2CD_INTR_ACK_RX_TX,
+ bus->base + ASPEED_I2C_INTR_STS_REG);
+ readl(bus->base + ASPEED_I2C_INTR_STS_REG);
+ }
+
irq_received &= ASPEED_I2CD_INTR_RECV_MASK;
irq_remaining = irq_received;

Thanks,
- Quan