serial: start_tx & buffer handling

[Muni Sekhar]

On Fri, May 4, 2018 at 5:44 PM, loïc tourlonias
<loic.tourlonias at gmail.com> wrote:
>  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.
Thanks a lot for this information.
>
>  HTH
>
>>>
>>>
>>> > thanks,
>>> >
>>> > greg k-h
>>>
>>>
>>>
>>> --
>>> Thanks,
>>> Sekhar
>>>
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>>
>>



-- 
Thanks,
Sekhar