linux-kernel - Re: [PATCH tty-next 2/2] serial: 8250: add driver for NI UARTs

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Message-ID: <163ef94e-4a3e-784d-b327-8e05a31c9f93@ni.com>
Date:   Fri, 31 Mar 2023 12:59:25 -0500
From:   Brenda Streiff <brenda.streiff@...com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Gratian Crisan <gratian.crisan@...com>,
        Jason Smith <jason.smith@...com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH tty-next 2/2] serial: 8250: add driver for NI UARTs

On 3/29/23 11:30, Greg Kroah-Hartman wrote:

Hi Greg, thanks for looking at this so quickly.

> On Wed, Mar 29, 2023 at 10:42:35AM -0500, Brenda Streiff wrote:
>> The National Instruments (NI) 16550 is a 16550-like UART with larger
>> FIFOs and embedded RS-232/RS-485 transceiver control circuitry. This
>> patch adds a driver that can operate this UART, which is used for
>> onboard serial ports in several NI embedded controller designs.
> 
> People are still making new 8250-like variants with different ways of
> controlling them these days?  That's the design pattern that will not
> die, or at least, it keeps getting a "value-add" :(
> 

This design has been on NI devices in some form since at least around
2009, so I hesitate to call it "new". But yes, it does seem like if you
let a hardware engineer be bored for too long you'll end up with a new
8250, a new SPI controller, or a new I2C controller. Sometimes all three!

>> +++ b/drivers/tty/serial/8250/8250_ni.c
>> @@ -0,0 +1,447 @@
>> +// SPDX-License-Identifier: GPL-2.0+
> 
> Do you really mean "+"?  Sorry, I have to ask.

It sits atop 8250_core.c which is marked as GPL-2.0+, and has been marked
as 'any later version' since it had been added to our tree [1] in the
pre-SPDX times.

[1] https://github.com/ni/linux/commit/6bf16de92cc9

>> +/*
>> + * NI 16550 UART Driver
>> + *
>> + * The National Instruments (NI) 16550 is a UART that is compatible with the
>> + * TL16C550C and OX16C950B register interfaces, but has additional functions
>> + * for RS-485 transceiver control. This driver implements support for the
>> + * additional functionality on top of the standard serial8250 core.
>> + *
>> + * Copyright 2012-2023 National Instruments Corporation
> 
> Um, 2012 and every year since then?  You all have had an out-of-tree
> driver for 11+ years that has been constantly updated every year?

It's been in _a_ tree-- NI's-- in some form for that long. But... yes,
this is correct.

I can't defend having it out of mainline for so long as having been a
good or wise choice, but that is the state of things.

>> +/* flags for ni16550_device_info */
>> +#define NI_HAS_PMR		BIT(0)
>> +
>> +struct ni16550_device_info {
>> +	unsigned int uartclk;
>> +	uint8_t prescaler;
> 
> Please use proper kernel types, u8.

Okay, did a scrub to remove C99 types.

> 
>> +	unsigned int flags;
> 
> And that's a horrible packing, do you mean to have those offsets?

I wasn't thinking about struct packing here, but yes these can be
reordered.

> 
> And why "unsigned int", don't you mean "u64" or "u32"?

For "uartclk" it was because struct uart_port::uartclk is an "unsigned
int" in include/linux/serial_8250.h.

For "flags" it was because there are loads of other places under
drivers/tty/serial/8250/* that use "flags" as an "unsigned int" or an
"unsigned long". Using a width-named types would be clearer (and I
don't mind making the change), but I tried to adhere to the convention
in nearby code.

>> +static int ni16550_rs485_config(struct uart_port *port,
>> +				struct ktermios *termios,
>> +				struct serial_rs485 *rs485)
>> +{
>> +	uint8_t pcr;
>> +	struct uart_8250_port *up = container_of(port, struct uart_8250_port,
>> +						 port);
>> +
>> +	/* "rs485" should be given to us non-NULL. */
>> +	if (WARN_ON(rs485 == NULL))
> 
> Can this ever happen?  If not, don't check for it, otherwise you just
> rebooted a machine that has panic-on-warn enabled :(
> 
>> +		return -EINVAL;
> 
> Or better yet, handle the case and return the error, why the WARN_ON()?

I'm not sure if this was ever possible, but it doesn't look like any of
the other drivers supporting rs485_config do this check today. Into the
dustbin it goes.