Commit f5bb8416 authored by Richard Genoud's avatar Richard Genoud Committed by Greg Kroah-Hartman

tty/serial: at91: fix hardware handshake on Atmel platforms

commit 9bcffe75 upstream.

After commit 1cf6e8fc ("tty/serial: at91: fix RTS line management
when hardware handshake is enabled"), the hardware handshake wasn't
functional anymore on Atmel platforms (beside SAMA5D2).

To understand why, one has to understand the flag ATMEL_US_USMODE_HWHS
first:
Before commit 1cf6e8fc ("tty/serial: at91: fix RTS line management
when hardware handshake is enabled"), this flag was never set.
Thus, the CTS/RTS where only handled by serial_core (and everything
worked just fine).

This commit introduced the use of the ATMEL_US_USMODE_HWHS flag,
enabling it for all boards when the user space enables flow control.

When the ATMEL_US_USMODE_HWHS is set, the Atmel USART controller
handles a part of the flow control job:
- disable the transmitter when the CTS pin gets high.
- drive the RTS pin high when the DMA buffer transfer is completed or
  PDC RX buffer full or RX FIFO is beyond threshold. (depending on the
  controller version).

NB: This feature is *not* mandatory for the flow control to work.
(Nevertheless, it's very useful if low latencies are needed.)

Now, the specifics of the ATMEL_US_USMODE_HWHS flag:

- For platforms with DMAC and no FIFOs (sam9x25, sam9x35, sama5D3,
sama5D4, sam9g15, sam9g25, sam9g35)* this feature simply doesn't work.
( source: https://lkml.org/lkml/2016/9/7/598 )
Tested it on sam9g35, the RTS pins always stays up, even when RXEN=1
or a new DMA transfer descriptor is set.
=> ATMEL_US_USMODE_HWHS must not be used for those platforms

- For platforms with a PDC (sam926{0,1,3}, sam9g10, sam9g20, sam9g45,
sam9g46)*, there's another kind of problem. Once the flag
ATMEL_US_USMODE_HWHS is set, the RTS pin can't be driven anymore via
RTSEN/RTSDIS in USART Control Register. The RTS pin can only be driven
by enabling/disabling the receiver or setting RCR=RNCR=0 in the PDC
(Receive (Next) Counter Register).
=> Doing this is beyond the scope of this patch and could add other
bugs, so the original (and working) behaviour should be set for those
platforms (meaning ATMEL_US_USMODE_HWHS flag should be unset).

- For platforms with a FIFO (sama5d2)*, the RTS pin is driven according
to the RX FIFO thresholds, and can be also driven by RTSEN/RTSDIS in
USART Control Register. No problem here.
(This was the use case of commit 1cf6e8fc ("tty/serial: at91: fix
RTS line management when hardware handshake is enabled"))
NB: If the CTS pin declared as a GPIO in the DTS, (for instance
cts-gpios = <&pioA PIN_PB31 GPIO_ACTIVE_LOW>), the transmitter will be
disabled.
=> ATMEL_US_USMODE_HWHS flag can be set for this platform ONLY IF the
CTS pin is not a GPIO.

So, the only case when ATMEL_US_USMODE_HWHS can be enabled is when
(atmel_use_fifo(port) &&
 !mctrl_gpio_to_gpiod(atmel_port->gpios, UART_GPIO_CTS))

Tested on all Atmel USART controller flavours:
AT91SAM9G35-CM (DMAC flavour), AT91SAM9G20-EK (PDC flavour),
SAMA5D2xplained (FIFO flavour).

* the list may not be exhaustive

Fixes: 1cf6e8fc ("tty/serial: at91: fix RTS line management when hardware handshake is enabled")
Signed-off-by: default avatarRichard Genoud <richard.genoud@gmail.com>
Acked-by: default avatarAlexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: default avatarCyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: default avatarUwe Kleine-König <u.kleine-koenig@pengutronix.de>
[nicolas.ferre@atmel.com: adapt to 4.4.x kernel for stable by adding
the atmel_port variable declaration which was missing]
Signed-off-by: default avatarNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 49163391
......@@ -2075,6 +2075,7 @@ static void atmel_serial_pm(struct uart_port *port, unsigned int state,
static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
struct ktermios *old)
{
struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
unsigned long flags;
unsigned int old_mode, mode, imr, quot, baud;
......@@ -2178,11 +2179,29 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
mode |= ATMEL_US_USMODE_RS485;
} else if (termios->c_cflag & CRTSCTS) {
/* RS232 with hardware handshake (RTS/CTS) */
if (atmel_use_dma_rx(port) && !atmel_use_fifo(port)) {
dev_info(port->dev, "not enabling hardware flow control because DMA is used");
termios->c_cflag &= ~CRTSCTS;
} else {
if (atmel_use_fifo(port) &&
!mctrl_gpio_to_gpiod(atmel_port->gpios, UART_GPIO_CTS)) {
/*
* with ATMEL_US_USMODE_HWHS set, the controller will
* be able to drive the RTS pin high/low when the RX
* FIFO is above RXFTHRES/below RXFTHRES2.
* It will also disable the transmitter when the CTS
* pin is high.
* This mode is not activated if CTS pin is a GPIO
* because in this case, the transmitter is always
* disabled (there must be an internal pull-up
* responsible for this behaviour).
* If the RTS pin is a GPIO, the controller won't be
* able to drive it according to the FIFO thresholds,
* but it will be handled by the driver.
*/
mode |= ATMEL_US_USMODE_HWHS;
} else {
/*
* For platforms without FIFO, the flow control is
* handled by the driver.
*/
mode |= ATMEL_US_USMODE_NORMAL;
}
} else {
/* RS232 without hadware handshake */
......
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