Hi On Fri, May 4, 2018 at 6:31 AM, Muni Sekhar <[email protected]> wrote:
> On Fri, May 4, 2018 at 12:04 AM, Greg KH <[email protected]> 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 > > _______________________________________________ > Kernelnewbies mailing list > [email protected] > https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies >
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