[PATCH] tty: rp2: Fix reset with non forgiving PCIe host bridges

[Florian Fainelli <florian.fainelli@broadcom.com>]

The write to RP2_GLOBAL_CMD followed by an immediate read of
RP2_GLOBAL_CMD in rp2_reset_asic() is intented to flush out the write,
however by then the device is already in reset and cannot respond to a
memory cycle access.

On platforms such as the Raspberry Pi 4 and others using the
pcie-brcmstb.c driver, any memory access to a device that cannot respond
is met with a fatal system error, rather than being substituted with all
1s as is usually the case on PC platforms.

Swapping the delay and the read ensures that the device has finished
resetting before we attempt to read from it.

Fixes: 7d9f49afa451 ("serial: rp2: New driver for Comtrol RocketPort 2 cards")
Suggested-by: Jim Quinlan <james.quinlan@broadcom.com>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
---
 drivers/tty/serial/rp2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/rp2.c b/drivers/tty/serial/rp2.c
index 4132fcff7d4e..8bab2aedc499 100644
--- a/drivers/tty/serial/rp2.c
+++ b/drivers/tty/serial/rp2.c
@@ -577,8 +577,8 @@ static void rp2_reset_asic(struct rp2_card *card, unsigned int asic_id)
 	u32 clk_cfg;
 
 	writew(1, base + RP2_GLOBAL_CMD);
-	readw(base + RP2_GLOBAL_CMD);
 	msleep(100);
+	readw(base + RP2_GLOBAL_CMD);
 	writel(0, base + RP2_CLK_PRESCALER);
 
 	/* TDM clock configuration */
-- 
2.43.0