From: Jamin Lin <jamin_lin@aspeedtech.com>

The interrupt status field is W1C, where a set bit on read indicates an
interrupt is pending. If the bit extracted from data is set it should
clear the corresponding bit in reg_value. However, if the extracted
bit is clear then the value of the corresponding bit in reg_value
should be unchanged.

SHARED_FIELD_EX32() extracts the interrupt status bit from the write
(data). reg_value is set to the set's interrupt status, which means
that for any pin with an interrupt pending, the corresponding bit is
set. The deposit32() call updates the bit at pin_idx in the
reg_value, using the value extracted from the write (data).

The result is that if multiple interrupt status bits
were pending and the write was acknowledging specific one bit,
then the all interrupt status bits will be cleared.
However, it is index mode and should only clear the corresponding bit.

For example, say we have an interrupt pending for GPIOA0, where the
following statements are true:

   set->int_status == 0b01
   s->pending == 1

Before it is acknowledged, an interrupt becomes pending for GPIOA1:

   set->int_status == 0b11
   s->pending == 2

A write is issued to acknowledge the interrupt for GPIOA0. This causes
the following sequence:

   reg_value == 0b11
   pending == 2
   s->pending == 0
   set->int_status == 0b00

It should only clear bit 0 in index mode and the correct result
should be as following.

   set->int_status == 0b11
   s->pending == 2

   pending == 1
   s->pending == 1
   set->int_status == 0b10

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Suggested-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
---
 hw/gpio/aspeed_gpio.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/hw/gpio/aspeed_gpio.c b/hw/gpio/aspeed_gpio.c
index 8725606aecae..16c18ea2f743 100644
--- a/hw/gpio/aspeed_gpio.c
+++ b/hw/gpio/aspeed_gpio.c
@@ -641,7 +641,7 @@ static void aspeed_gpio_write_index_mode(void *opaque, hwaddr offset,
     uint32_t pin_idx = reg_idx_number % ASPEED_GPIOS_PER_SET;
     uint32_t group_idx = pin_idx / GPIOS_PER_GROUP;
     uint32_t reg_value = 0;
-    uint32_t cleared;
+    uint32_t pending = 0;
 
     set = &s->sets[set_idx];
     props = &agc->props[set_idx];
@@ -703,16 +703,23 @@ static void aspeed_gpio_write_index_mode(void *opaque, hwaddr offset,
                               FIELD_EX32(data, GPIO_INDEX_REG, INT_SENS_2));
         set->int_sens_2 = update_value_control_source(set, set->int_sens_2,
                                                       reg_value);
-        /* set interrupt status */
-        reg_value = set->int_status;
-        reg_value = deposit32(reg_value, pin_idx, 1,
-                              FIELD_EX32(data, GPIO_INDEX_REG, INT_STATUS));
-        cleared = ctpop32(reg_value & set->int_status);
-        if (s->pending && cleared) {
-            assert(s->pending >= cleared);
-            s->pending -= cleared;
+        /* interrupt status */
+        if (FIELD_EX32(data, GPIO_INDEX_REG, INT_STATUS)) {
+            /* pending is either 1 or 0 for a 1-bit field */
+            pending = extract32(set->int_status, pin_idx, 1);
+
+            assert(s->pending >= pending);
+
+            /* No change to s->pending if pending is 0 */
+            s->pending -= pending;
+
+            /*
+             * The write acknowledged the interrupt regardless of whether it
+             * was pending or not. The post-condition is that it mustn't be
+             * pending. Unconditionally clear the status bit.
+             */
+            set->int_status = deposit32(set->int_status, pin_idx, 1, 0);
         }
-        set->int_status &= ~reg_value;
         break;
     case gpio_reg_idx_debounce:
         reg_value = set->debounce_1;
-- 
2.47.0