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2b4bac48c1
linux/drivers/tty/serial
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Jan Kundrát 2b4bac48c1 serial: max310x: Use batched reads when reasonably safe 
The hardware has a 128 byte RX FIFO buffer for each independent UART.
Previously, the code was always reading that byte-by-byte via
independent SPI transactions and the associated overhead. In practice,
this led to up to eight bytes over SPI for just one byte in the UART's
RX FIFO:

- reading the global IRQ register (two bytes, one for command, the other
for data)
- reading one UART's ISR (again two bytes)
- reading the byte count (two bytes yet again)
- finally, reading one byte of the FIFO via another two-byte transaction

We cannot always use a batched read. If the TTY is set to intercept
break conditions or report framing or parity errors, then it is required
to check the Line Status Register (LSR) for each byte which is read from
the RX FIFO. The documentation does not show a way of doing that in a
single SPI transaction; registers 0x00 and 0x04 are separate.

In my testing, this is no silver bullet. I was feeding 2MB of random
data over four daisy-chaned UARTs of MAX14830, and this is the
distribution that I was getting:

- R <= 1: 7437322
- R <= 2: 162093
- R <= 4: 4093
- R <= 8: 4196
- R <= 16: 645
- R <= 32: 165
- R <= 64: 58
- R <= 128: 0

For a reference, batching the write operations works much better:

- W <= 1: 2664
- W <= 2: 1305
- W <= 4: 627
- W <= 8: 371
- W <= 16: 121
- W <= 32: 68
- W <= 64: 33
- W <= 128: 63139

That's probably because this HW/SW combination (Clearfog Base, Armada
388) is probably "good enough" to react to the chip's IRQ "fast enough"
most of the time. Still, I was getting RX overruns every now and then.
In future, I plan to improve this by letting the RX FIFO be filled a
little more (the chip has support for that and also for a "stale
timeout" to prevent additional starvation).

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2017-12-19 09:59:02 +01:00
..
8250 serial: 8250: 8250_omap: Fix spelling error. 2017-12-15 20:27:44 +01:00
cpm_uart TTY/Serial patches for 4.15-rc1 2017-11-13 21:05:31 -08:00
jsm tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
21285.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
altera_jtaguart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
altera_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
amba-pl010.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
amba-pl011.c serial: pl011: Use cached copy of IMSC register 2017-11-28 15:35:51 +01:00
amba-pl011.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
apbuart.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
apbuart.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
ar933x_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
arc_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
atmel_serial.c serial: Make retrieval of rs485 properties platform-agnostic 2017-11-28 16:04:57 +01:00
atmel_serial.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
bcm63xx_uart.c MIPS changes for 4.15 2017-11-15 11:36:08 -08:00
bfin_sport_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
bfin_sport_uart.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
bfin_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
clps711x.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
crisv10.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
crisv10.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
digicolor-usart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
dz.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
dz.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
earlycon-arm-semihost.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
earlycon.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
efm32-uart.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
etraxfs-uart.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
fsl_lpuart.c serial: fsl_lpuart: Support common rs485 binding for RTS polarity 2017-11-28 16:05:20 +01:00
icom.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
icom.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
ifx6x60.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
ifx6x60.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
imx.c serial: imx: Support common rs485 binding for RTS polarity 2017-11-28 16:05:20 +01:00
ioc3_serial.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
ioc4_serial.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
ip22zilog.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
ip22zilog.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
Kconfig tty: serial: sh-sci: Add default for number of ports for compile-testing 2017-12-18 10:28:31 +01:00
kgdb_nmi.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
kgdboc.c Modules updates for v4.15 2017-11-15 13:46:33 -08:00
lantiq.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
lpc32xx_hs.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
m32r_sio_reg.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
m32r_sio.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
Makefile License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
max310x.c serial: max310x: Use batched reads when reasonably safe 2017-12-19 09:59:02 +01:00
max3100.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
mcf.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
men_z135_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
meson_uart.c tty: serial: meson: fix typo in the "stop bit" register definition 2017-11-28 15:32:33 +01:00
mpc52xx_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
mps2-uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
mpsc.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
msm_serial.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
mux.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
mvebu-uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
mxs-auart.c tty: serial: mxs-auart: fix error handling in mxs_auart_probe 2017-12-15 20:27:44 +01:00
netx-serial.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
omap-serial.c serial: core: Support common rs485 binding for RTS polarity 2017-11-28 16:05:19 +01:00
owl-uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
pch_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
pic32_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
pic32_uart.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
pmac_zilog.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
pmac_zilog.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
pnx8xxx_uart.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
pxa.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
rp2.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
sa1100.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
samsung.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
samsung.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
sb1250-duart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
sc16is7xx.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
sccnxp.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
serial_core.c serial: core: Support common rs485 binding for RTS polarity 2017-11-28 16:05:19 +01:00
serial_ks8695.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
serial_mctrl_gpio.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
serial_mctrl_gpio.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
serial_txx9.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
serial-tegra.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
sh-sci.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
sh-sci.h TTY/Serial patches for 4.15-rc1 2017-11-13 21:05:31 -08:00
sirfsoc_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
sirfsoc_uart.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
sn_console.c treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
sprd_serial.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
st-asc.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
stm32-usart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
stm32-usart.h serial: stm32: fix name conflict with 8250 2017-12-15 20:27:43 +01:00
suncore.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
sunhv.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
sunsab.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
sunsab.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
sunsu.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
sunzilog.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
sunzilog.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
tilegx.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
timbuart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
timbuart.h tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
uartlite.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
ucc_uart.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
vr41xx_siu.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
vt8500_serial.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
xilinx_uartps.c tty: serial: Remove redundant license text 2017-11-08 13:08:12 +01:00
zs.c tty: add SPDX identifiers to all remaining files in drivers/tty/ 2017-11-08 13:08:12 +01:00
zs.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00