serial: start_tx & buffer handling

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

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

Re: serial: start_tx & buffer handling

[Muni Sekhar]

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


> thanks,
>
> greg k-h



-- 
Thanks,
Sekhar

Re: serial: start_tx & buffer handling

[loïc tourlonias]

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Hi

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

> On Fri, May 4, 2018 at 12:04 AM, Greg KH <greg@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
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies@kernelnewbies.org
> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>

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Re: serial: start_tx & buffer handling

[loïc tourlonias]

 Hi

 On Fri, May 4, 2018 at 6:31 AM, Muni Sekhar <munisekharrms@gmail.com> wrote:
>>
>> On Fri, May 4, 2018 at 12:04 AM, Greg KH <greg@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
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies@kernelnewbies.org
>> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>

Re: serial: start_tx & buffer handling

[Muni Sekhar]

On Fri, May 4, 2018 at 5:44 PM, loïc tourlonias
<loic.tourlonias@gmail.com> wrote:
>  Hi
>
>  On Fri, May 4, 2018 at 6:31 AM, Muni Sekhar <munisekharrms@gmail.com> wrote:
>>>
>>> On Fri, May 4, 2018 at 12:04 AM, Greg KH <greg@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
>>>
>>> _______________________________________________
>>> Kernelnewbies mailing list
>>> Kernelnewbies@kernelnewbies.org
>>> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>>



-- 
Thanks,
Sekhar