Re: [PATCH] serial: 8250_aspeed_vuart: add PORT_ASPEED_VUART port type
From: Zev Weiss
Date: Thu Feb 10 2022 - 02:50:02 EST
On Wed, Feb 09, 2022 at 11:40:42PM PST, Greg Kroah-Hartman wrote:
On Wed, Feb 09, 2022 at 12:34:14PM -0800, Zev Weiss wrote:
Commit 54da3e381c2b ("serial: 8250_aspeed_vuart: use UPF_IOREMAP to
set up register mapping") fixed a bug that had, as a side-effect,
prevented the 8250_aspeed_vuart driver from enabling the VUART's
FIFOs. However, fixing that (and hence enabling the FIFOs) has in
turn revealed what appears to be a hardware bug in the ASPEED VUART in
which the host-side THRE bit doesn't get if the BMC-side receive FIFO
trigger level is set to anything but one byte. This causes problems
for polled-mode writes from the host -- for example, Linux kernel
console writes proceed at a glacial pace (less than 100 bytes per
second) because the write path waits for a 10ms timeout to expire
after every character instead of being able to continue on to the next
character upon seeing THRE asserted. (GRUB behaves similarly.)
As a workaround, introduce a new port type for the ASPEED VUART that's
identical to PORT_16550A as it had previously been using, but with
UART_FCR_R_TRIG_00 instead to set the receive FIFO trigger level to
one byte, which (experimentally) seems to avoid the problematic THRE
behavior.
Signed-off-by: Zev Weiss <zev@xxxxxxxxxxxxxxxxx>
Tested-by: Konstantin Aladyshev <aladyshev22@xxxxxxxxx>
Do we need a "Fixes:" tag here as well?
I was wondering the same -- I left it out because it didn't seem like it
was strictly a bug in the earlier commit that's really being fixed per
se, but perhaps that's an overly pedantic distinction. I can certainly
add it if you'd prefer.
---
drivers/tty/serial/8250/8250_aspeed_vuart.c | 2 +-
drivers/tty/serial/8250/8250_port.c | 8 ++++++++
include/uapi/linux/serial_core.h | 3 +++
3 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/8250/8250_aspeed_vuart.c b/drivers/tty/serial/8250/8250_aspeed_vuart.c
index 2350fb3bb5e4..c2cecc6f47db 100644
--- a/drivers/tty/serial/8250/8250_aspeed_vuart.c
+++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c
@@ -487,7 +487,7 @@ static int aspeed_vuart_probe(struct platform_device *pdev)
port.port.irq = irq_of_parse_and_map(np, 0);
port.port.handle_irq = aspeed_vuart_handle_irq;
port.port.iotype = UPIO_MEM;
- port.port.type = PORT_16550A;
+ port.port.type = PORT_ASPEED_VUART;
port.port.uartclk = clk;
port.port.flags = UPF_SHARE_IRQ | UPF_BOOT_AUTOCONF | UPF_IOREMAP
| UPF_FIXED_PORT | UPF_FIXED_TYPE | UPF_NO_THRE_TEST;
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 3b12bfc1ed67..973870ebff69 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -307,6 +307,14 @@ static const struct serial8250_config uart_config[] = {
.rxtrig_bytes = {1, 32, 64, 112},
.flags = UART_CAP_FIFO | UART_CAP_SLEEP,
},
+ [PORT_ASPEED_VUART] = {
+ .name = "ASPEED VUART",
+ .fifo_size = 16,
+ .tx_loadsz = 16,
+ .fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_00,
+ .rxtrig_bytes = {1, 4, 8, 14},
+ .flags = UART_CAP_FIFO,
+ },
};
/* Uart divisor latch read */
diff --git a/include/uapi/linux/serial_core.h b/include/uapi/linux/serial_core.h
index c4042dcfdc0c..cd11748833e6 100644
--- a/include/uapi/linux/serial_core.h
+++ b/include/uapi/linux/serial_core.h
@@ -274,4 +274,7 @@
/* Freescale LINFlexD UART */
#define PORT_LINFLEXUART 122
+/* ASPEED AST2x00 virtual UART */
+#define PORT_ASPEED_VUART 123
Why does this value have to be in a uapi header file? What userspace
tool is going to need this?
I only put it there because that was where all the other port type
constants were defined, and wondered the same thing about the lot of
them. Is there a userspace tool that makes use of any of these?
Zev