tty: uart: custom speed

[Vladimir Zapolskiy]

On 03/04/2019 05:23 PM, Subhashini Rao Beerisetty wrote:
> On Mon, Mar 4, 2019 at 8:27 PM Greg KH <gregkh at linuxfoundation.org> wrote:
>>
>> On Mon, Mar 04, 2019 at 08:19:44PM +0530, Subhashini Rao Beerisetty wrote:
>>> On Mon, Mar 4, 2019 at 6:53 PM Greg KH <gregkh at linuxfoundation.org> wrote:
>>>>
>>>> On Mon, Mar 04, 2019 at 05:46:54PM +0530, Subhashini Rao Beerisetty wrote:
>>>>>   Hi All,
>>>>>
>>>>>
>>>>> I’ve an UART ports on Xilinx FPGA board and it gets connected to PC
>>>>> via PCIe bus. I could not find any kernel serial driver which supports
>>>>> our hardware so I plan to develop a new driver. I see two approaches
>>>>> to develop an UART driver i.e. either by using tty_register_driver()
>>>>> or an uart_register_driver().
>>>>>
>>>>>
>>>>> Regarding my UART module, it has a counter of 16 bits and runs on a
>>>>> 32Mhz clock. It supports all the standard & non-standard baud’s up to
>>>>> 4Mbps.
>>>>
>>>> What type of UART is it?  Odds are it is based on an existing design, no
>>>> one creates a brand-new UART anymore.  Hopefully.  If not, what a
>>>> waste...
>>>
>>> For UART type, I see the permitted types are none, 8250, 16450, 16550,
>>> 16550A, 16650, 16650V2, 16654, 16750, 16850, 16950, and 16954 etc.
>>> Looking into the data sheet I haven’t found any register or parameter
>>> defining any of those UART types.  Is ‘UART type’ is determined from a
>>> register settings point of view or a pinout point of view?
>>
>> register settings point of view.
>>
>> I suggest you do some basic research into how UARTs work before
>> continuing with this effort, it will save you a lot of time in the end
>> :)
> 
> Thanks for you suggestion. I will study on this, I’m glad if you can
> point some good documentation links on this.
> 
>>
>>>>> If I used struct tty_operations, I noticed that baud rate changing is
>>>>> done via “.set_termios” API, but this method only supports standard
>>>>> baud rates. I’d like to know why this API does not support
>>>>> non-standard baud rates?
>>>>
>>>> Why do you think it does not?
>>> From data sheet point of view I'm clear on how to set the non-standard
>>> baud rate, even I exposed a custom Ioctl for this. But here I’m trying
>>> to understand how it is achieved by using available UART kernel
>>> framework.
>>> For non-standard baud rate requests, observed that
>>> tty->termios.c_ispeed & tty->termios.c_ospeed set to 38400. I did not
>>> understood why it changes?
>>
>> Have you read the documentation on how to set custom baud rates?  I
>> can't find the link to it at the moment, but it is very possible to do
>> that today, no special ioctls are needed at all.  I think someone was
>> finally working on getting glibc to support it directly, but I do not
>> know if those patches ever got merged, so you would just have to "open
>> code" it in userspace if you want to do this.
> 
> I could not find clear documentation on this, I’m very much thankful
> if someone point me on this. I thought of exploring TIOCGSERIAL and
> TIOCSSERIAL Ioctl’s to set custom baud rates. Now it looks like I
> should read the kernel code to understand how to achieve this without
> special ioctls.
> 

You should try to set a custom baudrate with help of TCSETS2:

----8<----

static int set_speed(int fd, speed_t speed)
{
	struct termios2 tio;
	int ret;

	ret = ioctl(fd, TCGETS2, &tio);
	if (ret) {
		printf("TCGETS2 failed: %d\n", ret);
		return ret;
	}

	tio.c_cflag &= ~CBAUD;
	tio.c_cflag |= BOTHER;
	tio.c_ispeed = speed;
	tio.c_ospeed = speed;

	ret = ioctl(fd, TCSETS2, &tio);
	if (ret) {
		printf("TCSETS2 failed to set speed %u: %d\n", speed, ret);
		return ret;
	}

	return 0;
}

----8<----

--
Best wishes,
Vladimir