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From:   Muni Sekhar <munisekharrms@gmail.com>
Date:   Fri, 4 May 2018 19:40:20 +0530
Message-ID: <CAHhAz+h9raK0qpuB2JEYY2TmNamt3oG3N0iUF-jjNUVboiaXdw@mail.gmail.com>
Subject: Re: serial: start_tx & buffer handling
To:     =?UTF-8?Q?lo=C3=AFc_tourlonias?= <loic.tourlonias@gmail.com>
Cc:     Greg KH <greg@kroah.com>, linux-kernel@vger.kernel.org,
        linux-serial <linux-serial@vger.kernel.org>,
        kernelnewbies <kernelnewbies@kernelnewbies.org>
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On Fri, May 4, 2018 at 5:44 PM, lo=C3=AFc tourlonias
<loic.tourlonias@gmail.com> wrote:
>  Hi
>
>  On Fri, May 4, 2018 at 6:31 AM, Muni Sekhar <munisekharrms@gmail.com> wr=
ote:
>>>
>>> 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=E2=80=99m 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 mo=
de
>>> >> write() API lands into kernel=E2=80=99s tty_write() ("drivers/tty/tt=
y_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=E2=80=99s start_tx() (struct uart_o=
ps
>>> >> .start_tx). But here I could not find how the buffer kept in circ_bu=
f
>>> >> is copied to serial port=E2=80=99s 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 doe=
s
>>> > 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 flus=
h
>>> > the buffer out to the hardware.
>>> >
>>>
>>> I=E2=80=99m looking for any existing sample code which does DMA transfe=
rs 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() functio=
n
>  called from uart_startup(). It's released in release_tx() function calle=
d
>  from uart_shutdown(). DMA buffer is managed in schedule_tx() function ca=
lled
>  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
>>
>>


--=20
Thanks,
Sekhar