Re: tty: uart: custom speed

[Greg KH <gregkh@linuxfoundation.org>]

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@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
:)

> > > 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.

good luck!

greg k-h

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