Re: [PATCH 00/10] Raspberry Pi SPI speedups

[Noralf Trønnes <noralf@xxxxxxxxxxx>]

Den 03.08.2019 12.10, skrev Lukas Wunner:
> So far the BCM2835 SPI driver cannot cope with TX-only and RX-only
> transfers (rx_buf or tx_buf is NULL) when using DMA:  It relies on
> the SPI core to convert them to full-duplex transfers by allocating
> and DMA-mapping a dummy rx_buf or tx_buf.  This costs performance.
> 
> Resolve by pre-allocating reusable DMA descriptors which cyclically
> clear the RX FIFO (for TX-only transfers) or zero-fill the TX FIFO
> (for RX-only transfers).  Patch [07/10] provides some numbers for
> the achieved latency improvement and CPU time reduction with an
> SPI Ethernet controller.  SPI displays should see a similar speedup.
> I've also made an effort to reduce peripheral and memory bus accesses.
> 
> The series is meant to be applied on top of broonie/for-next.
> It can be applied to Linus' current tree if commit
> 8d8bef503658 ("spi: bcm2835: Fix 3-wire mode if DMA is enabled")
> is cherry-picked from broonie's repo beforehand.
> 
> Please review and test.  Thank you.
> 

Tested-by: Noralf Trønnes <noralf@xxxxxxxxxxx>

I've tested on a 320x240 RGB565 display, flipping 2 framebuffers as fast
as possible. The buffers are 320x240x2=150kB which are maxsize split by
spi-bcm2835.

I see a small increase in speed from ~34.75 to ~35.55 fps using this series.

Details:

$ ~/libdrm/tests/modetest/modetest -M mi0283qt -s 31:320x240@RG16 -v
setting mode 320x240-0Hz@RG16 on connectors 31, crtc 34
freq: 30.36Hz
freq: 35.32Hz
freq: 35.56Hz
freq: 35.57Hz
freq: 35.54Hz
freq: 35.55Hz
freq: 35.60Hz
freq: 35.62Hz
freq: 35.50Hz
freq: 35.23Hz
freq: 35.49Hz
freq: 35.44Hz
freq: 35.39Hz
freq: 35.58Hz

$ od --endian big -tu4 -An
/sys/bus/spi/devices/spi0.0/of_node/spi-max-frequency
   64000000

<skip those that are zero>
pi@pi2835:~$ tail -n +1 /sys/bus/spi/devices/spi0.0/statistics/*
==> /sys/bus/spi/devices/spi0.0/statistics/bytes <==
131644720
==> /sys/bus/spi/devices/spi0.0/statistics/bytes_rx <==
2
==> /sys/bus/spi/devices/spi0.0/statistics/bytes_tx <==
131644718
==> /sys/bus/spi/devices/spi0.0/statistics/messages <==
5188
==> /sys/bus/spi/devices/spi0.0/statistics/spi_sync <==
5188
==> /sys/bus/spi/devices/spi0.0/statistics/spi_sync_immediate <==
5188
==> /sys/bus/spi/devices/spi0.0/statistics/transfer_bytes_histo_0-1 <==
2609
==>
/sys/bus/spi/devices/spi0.0/statistics/transfer_bytes_histo_16384-32767 <==
857
==> /sys/bus/spi/devices/spi0.0/statistics/transfer_bytes_histo_2-3 <==
5
==>
/sys/bus/spi/devices/spi0.0/statistics/transfer_bytes_histo_32768-65535 <==
1714
==> /sys/bus/spi/devices/spi0.0/statistics/transfer_bytes_histo_4-7 <==
1717
==> /sys/bus/spi/devices/spi0.0/statistics/transfer_bytes_histo_8-15 <==
2
==> /sys/bus/spi/devices/spi0.0/statistics/transfers <==
6904
==> /sys/bus/spi/devices/spi0.0/statistics/transfers_split_maxsize <==
857


Noralf.