tty: uart: custom speed

[Subhashini Rao Beerisetty]

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?

>
> > 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?

>
> Anyway, use the uart interface as odds are your uart is already
> supported.  Get the data sheet and start reading :)
>
> good luck,
>
> greg k-h