[Xen-devel] [PATCH v2 1/2] ns16550: Add compatible string for Raspberry Pi 4

[Stewart Hildebrand <stewart.hildebrand@xxxxxxxxxxxxxxx>]

Per the BCM2835 peripherals datasheet [1] page 10:
"The UART core is build to emulate 16550 behaviour ... The implemented
UART is not a 16650 compatible UART However as far as possible the
first 8 control and status registers are laid out like a 16550 UART. Al
16550 register bits which are not supported can be written but will be
ignored and read back as 0. All control bits for simple UART receive/
transmit operations are available."

Additionally, Linux uses the 8250/16550 driver for the aux UART [2].

Unfortunately the brcm,bcm2835-aux-uart device tree binding doesn't
have the reg-shift and reg-io-width properties [3]. Thus, the reg-shift
and reg-io-width properties are inherent properties of this UART.

Thanks to Andre Przywara for contributing the reg-shift and
reg-io-width setting snippet.

In my testing, I have relied on enable_uart=1 being set in config.txt,
a configuration file read by the Raspberry Pi's firmware. With
enable_uart=1, the firmware performs UART initialization.

[1] 
https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2835/BCM2835-ARM-Peripherals.pdf
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/tty/serial/8250/8250_bcm2835aux.c
[3] 
https://www.kernel.org/doc/Documentation/devicetree/bindings/serial/brcm,bcm2835-aux-uart.txt

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxxxxxxxxxx>
---
 xen/drivers/char/ns16550.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index e518f2d790..8667de6d67 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -1585,6 +1585,12 @@ static int __init ns16550_uart_dt_init(struct 
dt_device_node *dev,
     if ( uart->reg_width != 1 && uart->reg_width != 4 )
         return -EINVAL;
 
+    if ( dt_device_is_compatible(dev, "brcm,bcm2835-aux-uart") )
+    {
+        uart->reg_width = 4;
+        uart->reg_shift = 2;
+    }
+
     res = platform_get_irq(dev, 0);
     if ( ! res )
         return -EINVAL;
@@ -1611,6 +1617,7 @@ static const struct dt_device_match ns16550_dt_match[] 
__initconst =
     DT_MATCH_COMPATIBLE("ns16550"),
     DT_MATCH_COMPATIBLE("ns16550a"),
     DT_MATCH_COMPATIBLE("snps,dw-apb-uart"),
+    DT_MATCH_COMPATIBLE("brcm,bcm2835-aux-uart"),
     { /* sentinel */ },
 };
 
-- 
2.22.0


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel