Change in ...coreboot[master]: sifive uart: Add memory barrier instruction

[Xiang Wang (Code Review)]

Xiang Wang has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/29789

Change subject: sifive uart: Add memory barrier instruction
......................................................................

sifive uart: Add memory barrier instruction

If the memory barrier instruction is missing, the following code will be
optimized.

source code:
    do {
    v = *reg;
    } while (v & mask);

Optimized code:
    v = *reg;
    do {
    } while (v & mask);

In order to prevent such optimization, you need to add memory barrier
instructions, the source code is modified as follows.
    do {
    v = *reg;
    asm volatile ("":::"memory");
    } while (v & mask);

Change-Id: I7fdf178485af1ef0fd8d10f1f4919b9e31284457
Signed-off-by: Xiang Wang 
---
M src/drivers/uart/sifive.c
1 file changed, 7 insertions(+), 7 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/89/29789/1

diff --git a/src/drivers/uart/sifive.c b/src/drivers/uart/sifive.c
index ce8ead0..1a7370c 100644
--- a/src/drivers/uart/sifive.c
+++ b/src/drivers/uart/sifive.c
@@ -66,17 +66,15 @@
    sifive_uart_init(uart_platform_baseptr(idx), div);
 }

-static bool uart_can_tx(struct sifive_uart_registers *regs)
-{
-	return !(read32(&regs->txdata) & TXDATA_FULL);
-}
-
 void uart_tx_byte(int idx, unsigned char data)
 {
    struct sifive_uart_registers *regs = uart_platform_baseptr(idx);
+	uint32_t txdata;

-	while (!uart_can_tx(regs))
-		; /* TODO: implement a timeout */
+	do {
+		txdata = read32(regs->txdata);
+		asm volatile ("":::"memory");
+	} while (txdata & TXDATA_FULL)

write32(&regs->txdata, data);
 }
@@ -89,6 +87,7 @@
    /* Use the TX watermark bit to find out if the TX FIFO is empty */
    do {
    	ip = read32(&regs->ip);
+		asm volatile ("":::"memory");
    } while (!(ip & IP_TXWM));
 }

@@ -99,6 +98,7 @@

do {
    	rxdata = read32(&regs->rxdata);
+		asm volatile ("":::"memory");
    } while (rxdata & RXDATA_EMPTY);

return rxdata & 0xff;