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

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Date:   Fri, 16 Sep 2016 15:47:41 -0500
From:   Thor Thayer <tthayer@...nsource.altera.com>
To:     Rob Herring <robh@...nel.org>
CC:     <gregkh@...uxfoundation.org>, <mark.rutland@....com>,
        <lftan@...era.com>, <jslaby@...e.com>, <arnd@...db.de>,
        <peter@...leysoftware.com>, <andriy.shevchenko@...ux.intel.com>,
        <anton.wuerfel@....de>, <phillip.raffeck@....de>,
        <yegorslists@...glemail.com>, <matwey@....msu.ru>,
        <mail@...iej.szmigiero.name>, <peter.ujfalusi@...com>,
        <jonathanh@...dia.com>, <dongsheng.wang@....com>,
        <luis@...ethencourt.com>, <linux-kernel@...r.kernel.org>,
        <linux-serial@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <tthayer.linux@...il.com>, <nios2-dev@...ts.rocketboards.org>
Subject: Re: [PATCH 1/4] Documentation: dt: serial: Add TX FIFO load size

Hi Rob,

On 09/16/2016 02:19 PM, Rob Herring wrote:
> On Thu, Sep 08, 2016 at 11:12:18AM -0500, tthayer@...nsource.altera.com wrote:
>> From: Thor Thayer <tthayer@...nsource.altera.com>
>>
>> Add the device tree bindings needed to support the TX FIFO
>> load size.
>>
>> Signed-off-by: Thor Thayer <tthayer@...nsource.altera.com>
>> ---
>>  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
> thresholds.
>
> Rob
>

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.

Thor