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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
..
accessibility
acpi ACPI / CPPC: Fix KASAN global out of bounds warning 2017-12-05 23:25:49 +01:00
amba A couple of dma-mapping updates: 2017-11-14 16:54:12 -08:00
android ANDROID: binder: fix transaction leak. 2017-11-28 16:54:45 +01:00
ata Merge branch 'for-4.15' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/libata 2017-11-15 14:11:41 -08:00
atm Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2017-11-29 13:10:25 -08:00
auxdisplay auxdisplay: img-ascii-lcd: Only build on archs that have IOMEM 2017-11-27 12:36:45 -08:00
base Power management fix for v4.15-rc3 2017-12-08 09:58:10 -08:00
bcma Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2017-11-15 11:56:19 -08:00
block Merge branch 'for-linus' of git://git.kernel.dk/linux-block 2017-12-01 08:05:45 -05:00
bluetooth
bus bus: arm-ccn: fix module unloading Error: Removing state 147 which has instances left. 2017-12-04 17:15:20 +00:00
cdrom Merge branch 'for-4.15/block' of git://git.kernel.dk/linux-block 2017-11-14 15:32:19 -08:00
char Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-26 14:11:54 -08:00
clk We have two changes to the core framework this time around. The first being a 2017-11-17 20:04:24 -08:00
clocksource - final batch of "non trivial" timer conversions (multi-tree dependencies, 2017-11-23 16:29:05 +01:00
connector
cpufreq cpufreq: mediatek: add missing MODULE_DESCRIPTION/AUTHOR/LICENSE 2017-11-22 00:00:14 +01:00
cpuidle powerpc updates for 4.15 2017-11-16 12:47:46 -08:00
crypto powerpc updates for 4.15 2017-11-16 12:47:46 -08:00
dax device-dax: implement ->split() to catch invalid munmap attempts 2017-11-29 18:40:42 -08:00
dca
devfreq
dio
dma dmaengine updates for 4.15-rc1 2017-11-14 16:49:31 -08:00
dma-buf Tracing updates for 4.15: 2017-11-17 14:58:01 -08:00
edac Modules updates for v4.15 2017-11-15 13:46:33 -08:00
eisa
extcon USB/PHY patches for 4.15-rc1 2017-11-13 21:14:07 -08:00
firewire Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-13 17:56:58 -08:00
firmware ARM: SoC fixes for 4.15-rc 2017-12-10 08:26:59 -08:00
fmc
fpga Char/Misc patches for 4.15-rc1 2017-11-16 09:10:59 -08:00
fsi
gpio gpio: pca953x: fix vendor prefix for PCA9654 2017-12-02 22:41:43 +01:00
gpu Merge tag 'drm-misc-fixes-2017-12-07' of git://anongit.freedesktop.org/drm/drm-misc into drm-fixes 2017-12-08 08:17:53 +10:00
hid treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
hsi HSI changes for the v4.15 series 2017-11-15 13:35:43 -08:00
hv Drivers: hv: vmbus: Fix a rescind issue 2017-11-28 16:56:26 +01:00
hwmon hwmon: (jc42) optionally try to disable the SMBUS timeout 2017-11-30 13:12:44 -08:00
hwspinlock hwspinlock update for v4.15 2017-11-17 20:16:20 -08:00
hwtracing Char/Misc patches for 4.15-rc1 2017-11-16 09:10:59 -08:00
i2c i2c: i2c-boardinfo: fix memory leaks on devinfo 2017-11-27 19:14:29 +01:00
ide Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/ide 2017-11-19 08:04:41 -10:00
idle
iio iio: health: max30102: Temperature should be in milli Celsius 2017-12-02 11:15:14 +00:00
infiniband Here is the first rc pull request for RDMA. This includes an important core 2017-12-05 10:10:15 -08:00
input treewide: Remove TIMER_FUNC_TYPE and TIMER_DATA_TYPE casts 2017-11-21 16:35:54 -08:00
iommu IOMMU fixes for Linux v4.15-rc3 2017-12-06 10:53:02 -08:00
ipack
irqchip Merge branch 'irq-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-26 14:39:20 -08:00
isdn treewide: setup_timer() -> timer_setup() (2 field) 2017-11-21 15:57:09 -08:00
leds LED updates for 4.15rc1 2017-11-14 18:09:31 -08:00
lightnvm lightnvm: Convert timers to use timer_setup() 2017-11-21 15:46:44 -08:00
macintosh Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-13 17:56:58 -08:00
mailbox Change to POLL api and fixes for FlexRM and OMAP driver 2017-11-15 13:39:18 -08:00
mcb
md md/raid1/10: add missed blk plug 2017-12-01 12:19:48 -08:00
media media fixes for v4.15-rc3 2017-12-08 13:18:47 -08:00
memory ARM: SoC driver updates for v4.15 2017-11-16 16:05:01 -08:00
memstick treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
message Modules updates for v4.15 2017-11-15 13:46:33 -08:00
mfd treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
misc Merge branch 'i2c/for-current' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux 2017-12-03 10:48:24 -05:00
mmc MMC core: 2017-12-01 08:14:22 -05:00
mtd Rename superblock flags (MS_xyz -> SB_xyz) 2017-11-27 13:05:09 -08:00
mux
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2017-12-08 13:32:44 -08:00
nfc treewide: setup_timer() -> timer_setup() (2 field) 2017-11-21 15:57:09 -08:00
ntb treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
nubus
nvdimm libnvdimm for 4.15 2017-11-17 09:51:57 -08:00
nvme nvme-pci: fix NULL pointer dereference in nvme_free_host_mem() 2017-11-28 08:49:26 -08:00
nvmem nvmem: meson-mx-efuse: fix reading from an offset other than 0 2017-12-19 09:56:03 +01:00
of of: overlay: Make node skipping in init_overlay_changeset() clearer 2017-12-08 09:32:18 -06:00
opp
oprofile
parisc
parport Char/Misc patches for 4.15-rc1 2017-11-16 09:10:59 -08:00
pci Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-26 14:11:54 -08:00
pcmcia drivers/pcmcia/sa1111_badge4.c: avoid unused function warning 2017-11-17 16:10:04 -08:00
perf arm64 updates for 4.15 2017-11-15 10:56:56 -08:00
phy USB/PHY patches for 4.15-rc1 2017-11-13 21:14:07 -08:00
pinctrl pinctrl: sunxi: Disable strict mode for H5 driver 2017-11-30 16:50:43 +01:00
platform Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-25 08:37:16 -10:00
pnp
power power supply and reset changes for the v4.15 series 2017-11-15 13:37:15 -08:00
powercap
pps treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
ps3
ptp xen: features and fixes for v4.15-rc1 2017-11-16 13:06:27 -08:00
pwm pwm: Changes for v4.15-rc1 2017-11-22 21:09:18 -10:00
rapidio Merge branch 'akpm' (patches from Andrew) 2017-11-17 16:56:17 -08:00
ras Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-13 17:56:58 -08:00
regulator - New Drivers 2017-11-16 09:15:57 -08:00
remoteproc remoteproc updates for v4.15 2017-11-17 20:14:10 -08:00
reset ARM: SoC driver updates for v4.15 2017-11-16 16:05:01 -08:00
rpmsg rpmsg updates for v4.15 2017-11-17 20:12:08 -08:00
rtc Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-25 08:37:16 -10:00
s390 s390: add a few more SPDX identifiers 2017-12-05 07:51:09 +01:00
sbus Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/sparc 2017-11-17 20:21:44 -08:00
scsi SCSI fixes on 20171205 2017-12-05 10:31:32 -08:00
sfi
sh A couple of dma-mapping updates: 2017-11-14 16:54:12 -08:00
sn
soc meson-gx-socinfo: Fix package id parsing 2017-11-30 15:29:44 -08:00
spi
spmi
ssb
staging Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2017-12-08 13:32:44 -08:00
target Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-11-25 08:37:16 -10:00
tc
tee optee: fix invalid of_node_put() in optee_driver_init() 2017-11-29 10:24:57 +01:00
thermal Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/rzhang/linux 2017-11-17 14:31:27 -08:00
thunderbolt Char/Misc patches for 4.15-rc1 2017-11-16 09:10:59 -08:00
tty serial: max310x: Use batched reads when reasonably safe 2017-12-19 09:59:02 +01:00
uio
usb usb: xhci: fix panic in xhci_free_virt_devices_depth_first 2017-12-01 15:24:59 +00:00
uwb treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
vfio VFIO Updates for Linux v4.15 2017-11-14 16:47:47 -08:00
vhost vhost: fix skb leak in handle_rx() 2017-12-02 21:31:03 -05:00
video fbdev changes for v4.15: 2017-11-20 21:50:24 -10:00
virt
virtio virtio_mmio: add cleanup for virtio_mmio_remove 2017-12-07 18:30:50 +02:00
vlynq
vme Char/Misc patches for 4.15-rc1 2017-11-16 09:10:59 -08:00
w1 Char/Misc patches for 4.15-rc1 2017-11-16 09:10:59 -08:00
watchdog treewide: setup_timer() -> timer_setup() 2017-11-21 15:57:07 -08:00
xen xen: fixes for 4.15-rc3 2017-12-08 12:53:43 -08:00
zorro
Kconfig
Makefile usb: build drivers/usb/common/ when USB_SUPPORT is set 2017-11-28 15:17:49 +01:00