tty: uart: custom speed

[Subhashini Rao Beerisetty]

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.

>
> good luck!
>
> greg k-h