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authorRichard Genoud <richard.genoud@gmail.com>2016-10-27 18:04:06 +0200
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-10-28 08:10:48 -0400
commit9bcffe7575b721d7b6d9b3090fe18809d9806e78 (patch)
tree8dfe52643778499b773b6b11f33a99195378a1b1 /drivers/tty
parent009e39ae44f4191188aeb6dfbf661b771dbbe515 (diff)
downloadlinux-9bcffe7575b721d7b6d9b3090fe18809d9806e78.tar.bz2
tty/serial: at91: fix hardware handshake on Atmel platforms
After commit 1cf6e8fc8341 ("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 1cf6e8fc8341 ("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 1cf6e8fc8341 ("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 Cc: <stable@vger.kernel.org> #4.4+ (beware, missing atmel_port variable) Fixes: 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management when hardware handshake is enabled") Signed-off-by: Richard Genoud <richard.genoud@gmail.com> Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com> Acked-by: Cyrille Pitchen <cyrille.pitchen@atmel.com> Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers/tty')
-rw-r--r--drivers/tty/serial/atmel_serial.c26
1 files changed, 22 insertions, 4 deletions
diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index fd8aa1f4ba78..168b10cad47b 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -2132,11 +2132,29 @@ 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 */