Re: [PATCH 1/4] Documentation: dt: serial: Add TX FIFO load size

From: Thor Thayer
Date: Fri Sep 16 2016 - 16:42:39 EST

Hi Rob,

On 09/16/2016 02:19 PM, Rob Herring wrote:
On Thu, Sep 08, 2016 at 11:12:18AM -0500, tthayer@xxxxxxxxxxxxxxxxxxxxx wrote:
From: Thor Thayer <tthayer@xxxxxxxxxxxxxxxxxxxxx>

Add the device tree bindings needed to support the TX FIFO
load size.

Signed-off-by: Thor Thayer <tthayer@xxxxxxxxxxxxxxxxxxxxx>
Documentation/devicetree/bindings/serial/8250.txt | 1 +
1 file changed, 1 insertion(+)

diff --git a/Documentation/devicetree/bindings/serial/8250.txt b/Documentation/devicetree/bindings/serial/8250.txt
index f5561ac..93de5ef 100644
--- a/Documentation/devicetree/bindings/serial/8250.txt
+++ b/Documentation/devicetree/bindings/serial/8250.txt
@@ -45,6 +45,7 @@ Optional properties:
- {rts,cts,dtr,dsr,rng,dcd}-gpios: specify a GPIO for RTS/CTS/DTR/DSR/RI/DCD
line respectively. It will use specified GPIO instead of the peripheral
function pin for the UART feature. If unsure, don't specify this property.
+- tx-loadsz: the number of bytes that can be loaded into the TX FIFO.

It seems you don't really need to know the loadsz, but just need the
threshold trigger level. So just use that directly. And please check if
any other UARTs have threshold properties. i.MX has programmable


Yes, it is correct that I only really need the loadsz, but there is a tx-loadsz parameter in the uart_8250_port structure. Since there currently isn't a way to update the default value of tx-loadsz from device tree, I thought this would be useful for other 8250 UARTs as well.

Yes, there are a couple of 8250 compatible UARTs that have programmable thresholds. The SC16654 and SC16C650 are 2 examples. However, I didn't see the programmable TX threshold implemented.

I looked at the i.MX UARTs but I don't see a programmable threshold parameter in the bindings( Documentation/devicetree/bindings/serial/fsl-imx-uart.txt).

I'm assuming you're saying that I should use the same binding names for this transmit threshold if they are available?

Thank you for reviewing and for the comments.