serial: start_tx & buffer handling

[loïc tourlonias]

Hi

On Fri, May 4, 2018 at 6:31 AM, Muni Sekhar <munisekharrms at gmail.com> wrote:

> On Fri, May 4, 2018 at 12:04 AM, Greg KH <greg at kroah.com> wrote:
> > On Thu, May 03, 2018 at 08:08:48PM +0530, Muni Sekhar wrote:
> >> Hi All,
> >>
> >> I’m trying to understand how user mode buffer is written to low level
> >> serial hardware registers.
> >>
> >> For this I read the kernel code and I came to know that from user mode
> >> write() API lands into kernel’s tty_write() ("drivers/tty/tty_io.c")
> >> and then it calls a uart_write() ("drivers/tty/serial/serial_core.c").
> >>
> >> In uart_write(), the buffer is copied to circ_buf and then it calls
> >> low level serial hardware driver’s start_tx() (struct uart_ops
> >> .start_tx). But here I could not find how the buffer kept in circ_buf
> >> is copied to serial port’s TX_FIFO registers?
> >>
> >> Can someone take a moment to explain me on this?
> >
> > It all depends on which specific UART driver you are looking at, they
> > all do it a bit different depending on the hardware.
> >
> > Which one are you looking at?  Look at what the start_tx callback does
> > for that specific driver, that should give you a hint as to how data
> > starts flowing.  Usually an interrupt is enabled that is used to flush
> > the buffer out to the hardware.
> >
>
> I’m looking for any existing sample code which does DMA transfers of
> UART transmitted data. I looked at the bcm63xx_uart.c, it looks it
> does not handle DMA transfers. Even copying the Tx buffer (from
> circ_buf) to UART_FIFO_REG happening in ISR.
>

You can have a look at atmel_serial kernel module (built for ARM).
https://elixir.bootlin.com/linux/latest/source/drivers/tty/serial/atmel_serial.c

The dma buffer is linked to uart circular buffer in prepare_tx() function
called from uart_startup(). It's released in release_tx() function called
from uart_shutdown(). DMA buffer is managed in schedule_tx() function
called from a tasklet triggered by the ISR.

HTH

>
>
> > thanks,
> >
> > greg k-h
>
>
>
> --
> Thanks,
> Sekhar
>
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