dect
/
linux-2.6
Archived
13
0
Fork 0
Code Releases Activity

serial: pl011: handle corruption at high clock speeds 

This works around a few glitches in the ST version of the PL011
serial driver when using very high baud rates, as we do in the
Ux500: 3, 3.25, 4 and 4.05 Mbps.

Problem Observed/rootcause:

When using high baud-rates, and the baudrate*8 is getting close to
the provided clock frequency (so a division factor close to 1), when
using bursts of characters (so they are abutted), then it seems as if
there is not enough time to detect the beginning of the start-bit which
is a timing reference for the entire character, and thus the sampling
moment of character bits is moving towards the end of each bit, instead
of the middle.

Fix:
Increase slightly the RX baud rate of the UART above the theoretical
baudrate by 5%. This will definitely give more margin time to the
UART_RX to correctly sample the data at the middle of the bit period.

Also fix the ages old copy-paste error in the very stressed comment,
it's referencing the registers used in the PL010 driver rather than
the PL011 ones.

Signed-off-by: Guillaume Jaunet <guillaume.jaunet@stericsson.com>
Signed-off-by: Christophe Arnal <christophe.arnal@stericsson.com>
Signed-off-by: Matthias Locher <matthias.locher@stericsson.com>
Signed-off-by: Rajanikanth HV <rajanikanth.hv@stericsson.com>
Cc: stable <stable@vger.kernel.org>
Cc: Bibek Basu <bibek.basu@stericsson.com>
Cc: Par-Gunnar Hjalmdahl <par-gunnar.hjalmdahl@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Linus Walleij 2012-09-26 17:21:36 +02:00 committed by Greg Kroah-Hartman
parent 91f61ce24f
commit c5dd553b9f
1 changed files with 14 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
drivers/tty/serial/amba-pl011.c
View File

@ -1595,13 +1595,26 @@ pl011_set_termios(struct uart_port *port, struct ktermios *termios,
old_cr &= ~ST_UART011_CR_OVSFACT;
}
/*
* Workaround for the ST Micro oversampling variants to
* increase the bitrate slightly, by lowering the divisor,
* to avoid delayed sampling of start bit at high speeds,
* else we see data corruption.
*/
if (uap->vendor->oversampling) {
if ((baud >= 3000000) && (baud < 3250000) && (quot > 1))
quot -= 1;
else if ((baud > 3250000) && (quot > 2))
quot -= 2;
}
/* Set baud rate */
writew(quot & 0x3f, port->membase + UART011_FBRD);
writew(quot >> 6, port->membase + UART011_IBRD);
/*
* ----------v----------v----------v----------v-----
* NOTE: MUST BE WRITTEN AFTER UARTLCR_M & UARTLCR_L
* NOTE: lcrh_tx and lcrh_rx MUST BE WRITTEN AFTER
* UART011_FBRD & UART011_IBRD.
* ----------^----------^----------^----------^-----
*/
writew(lcr_h, port->membase + uap->lcrh_rx);