jtag/drivers: bcm2835gpio: implement memory barriers when bitbashing

This GPIO driver is common to SoCs that have in-order ARM cores
(BCM2835) as well as superscalar (BCM2836-7) and speculative
out-of-order cores (BCM2711).

For BCM2837 and BCM2711, the processor can dual-issue stores and
is free to merge writes to peripheral memory for pages mapped
MT_NORMAL_NC, which is the default provided by /dev/[gpio]mem.

This can cause glitches (or missing edges) on GPIO pins when
toggled with no delay, as pipelined writes to the same address
can get arbitrarily squelched.

To prevent this happening, make sure the preceding write ops are
flushed outside the shareable domain by using a memory barrier.

Signed-off-by: Jonathan Bell <[email protected]>
Change-Id: I8805cc0911667bcb9b7f4ca340d7f4f1cb25d096
Reviewed-on: https://review.openocd.org/c/openocd/+/7258
Tested-by: jenkins
Reviewed-by: Antonio Borneo <[email protected]>

Author: P33M

src/jtag/drivers/bcm2835gpio.c

@@ -57,6 +57,15 @@ static struct initial_gpio_state {
 } initial_gpio_state[ADAPTER_GPIO_IDX_NUM];
 static uint32_t initial_drive_strength_etc;
 
+static inline void bcm2835_gpio_synchronize(void)
+{
+	/* Ensure that previous writes to GPIO registers are flushed out of
+	 * the inner shareable domain to prevent pipelined writes to the
+	 * same address being merged.
+	 */
+	__sync_synchronize();
+}
+
 static bool is_gpio_config_valid(enum adapter_gpio_config_index idx)
 {
 	/* Only chip 0 is supported, accept unset value (-1) too */
@@ -96,6 +105,7 @@ static void set_gpio_value(const struct adapter_gpio_config *gpio_config, int va
 		}
 		break;
 	}
+	bcm2835_gpio_synchronize();
 }
 
 static void restore_gpio(enum adapter_gpio_config_index idx)
@@ -109,6 +119,7 @@ static void restore_gpio(enum adapter_gpio_config_index idx)
 				GPIO_CLR = 1 << adapter_gpio_config[idx].gpio_num;
 		}
 	}
+	bcm2835_gpio_synchronize();
 }
 
 static void initialize_gpio(enum adapter_gpio_config_index idx)
@@ -143,6 +154,7 @@ static void initialize_gpio(enum adapter_gpio_config_index idx)
 	/* Direction for non push-pull is already set by set_gpio_value() */
 	if (adapter_gpio_config[idx].drive == ADAPTER_GPIO_DRIVE_MODE_PUSH_PULL)
 		OUT_GPIO(adapter_gpio_config[idx].gpio_num);
+	bcm2835_gpio_synchronize();
 }
 
 static bb_value_t bcm2835gpio_read(void)
@@ -164,6 +176,7 @@ static int bcm2835gpio_write(int tck, int tms, int tdi)
 
 	GPIO_SET = set;
 	GPIO_CLR = clear;
+	bcm2835_gpio_synchronize();
 
 	for (unsigned int i = 0; i < jtag_delay; i++)
 		asm volatile ("");
@@ -184,6 +197,7 @@ static int bcm2835gpio_swd_write_fast(int swclk, int swdio)
 
 	GPIO_SET = set;
 	GPIO_CLR = clear;
+	bcm2835_gpio_synchronize();
 
 	for (unsigned int i = 0; i < jtag_delay; i++)
 		asm volatile ("");
@@ -234,6 +248,7 @@ static void bcm2835_swdio_drive(bool is_output)
 		if (is_gpio_config_valid(ADAPTER_GPIO_IDX_SWDIO_DIR))
 			set_gpio_value(&adapter_gpio_config[ADAPTER_GPIO_IDX_SWDIO_DIR], 0);
 	}
+	bcm2835_gpio_synchronize();
 }
 
 static int bcm2835_swdio_read(void)