Re: serial: start_tx & buffer handling

[Muni Sekhar]

On Fri, May 4, 2018 at 12:04 AM, Greg KH  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.


> thanks,
>
> greg k-h



-- 
Thanks,
Sekhar

___
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

Re: serial: start_tx & buffer handling

[Greg KH]

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.

thanks,

greg k-h

___
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

serial: start_tx & buffer handling

[Muni Sekhar]

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?



-- 
Thanks,
Sekhar

___
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

serial: custom baud rate

[Muni Sekhar]

Hi All,

>From include/asm-generic/termbits.h , I see baudrate can be one of the
standard values: 50, 75, 110, 134, 150, 200, 300, 600, 1200, 1800,
2400, 4800, 9600, 19200, 38400, 57600, 115200, 230400, 460800, 50,
576000, 921600, 100, 1152000, 150, 200, 250, 300,
350, 400.


If I need to set a custom baud rates(e.g. 14400, 128000, 256000), does
Linux serial framework has any supporting method?

-- 
Thanks,
Sekhar

___
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies