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serial: 8250: Define RX trigger levels for OxSemi 950 devices 

[ Upstream commit d7aff291d069c4418285f3c8ee27b0ff67ce5998 ]

Oxford Semiconductor 950 serial port devices have a 128-byte FIFO and in
the enhanced (650) mode, which we select in `autoconfig_has_efr' with
the ECB bit set in the EFR register, they support the receive interrupt
trigger level selectable with FCR bits 7:6 from the set of 16, 32, 112,
120.  This applies to the original OX16C950 discrete UART[1] as well as
950 cores embedded into more complex devices.

For these devices we set the default to 112, which sets an excessively
high level of 112 or 7/8 of the FIFO capacity, unlike with other port
types where we choose at most 1/2 of their respective FIFO capacities.
Additionally we don't make the trigger level configurable.  Consequently
frequent input overruns happen with high bit rates where hardware flow
control cannot be used (e.g. terminal applications) even with otherwise
highly-performant systems.

Lower the default receive interrupt trigger level to 32 then, and make
it configurable.  Document the trigger levels along with other port
types, including the set of 16, 32, 64, 112 for the transmit interrupt
as well[2].

References:

[1] "OX16C950 rev B High Performance UART with 128 byte FIFOs", Oxford
    Semiconductor, Inc., DS-0031, Sep 05, Table 10: "Receiver Trigger
    Levels", p. 22

[2] same, Table 9: "Transmit Interrupt Trigger Levels", p. 22

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2106260608480.37803@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Maciej W. Rozycki 2021-06-26 06:11:51 +02:00 committed by Greg Kroah-Hartman
parent b050848bba
commit 5418023f81
2 changed files with 3 additions and 1 deletions
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3
drivers/tty/serial/8250/8250_port.c
View File

@ -125,7 +125,8 @@ static const struct serial8250_config uart_config[] = {
.name = "16C950/954",
.fifo_size = 128,
.tx_loadsz = 128,
.fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_10,
.fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_01,
.rxtrig_bytes = {16, 32, 112, 120},
/* UART_CAP_EFR breaks billionon CF bluetooth card. */
.flags = UART_CAP_FIFO | UART_CAP_SLEEP,
},

1
include/uapi/linux/serial_reg.h
View File

@ -62,6 +62,7 @@
* ST16C654: 8 16 56 60 8 16 32 56 PORT_16654
* TI16C750: 1 16 32 56 xx xx xx xx PORT_16750
* TI16C752: 8 16 56 60 8 16 32 56
* OX16C950: 16 32 112 120 16 32 64 112 PORT_16C950
* Tegra: 1 4 8 14 16 8 4 1 PORT_TEGRA
*/
#define UART_FCR_R_TRIG_00 0x00