WLAN from SDIO to SPI: sneak preview

Werner Almesberger werner at openmoko.org
Thu Sep 4 10:17:56 CEST 2008

The MMC-SPI version using the bit-banging GPIO driver is now working
reasonably well. I put a sneak preview here:


This is the slowest of all the possible stack combinations, so it has
more academic value than anything else.

Some performance data:

- ping -c 25 to my WRT54G

  25 packets transmitted, 24 packets received, 4% packet loss
  round-trip min/avg/max = 26.147/211.685/685.817 ms

- an SCP of the kernel from a PC connected via the WRT54G:

  uImage                                        100% 1678KB  19.5KB/s   01:26

The throughput is actually better than I expected ;-) The ping
latency came as a surprise, though. However, when I look at what
the competition is doing (the original stack):

- ping -c 25:

  25 packets transmitted, 25 packets received, 0% packet loss
  round-trip min/avg/max = 16.972/150.415/452.088 ms

- same SCP:

  uImage                                        100% 1678KB  88.3KB/s   00:19

That should be a lot better - my driver only gets a clock of about
300kHz while burning 90% CPU in spi_s3c24xx_gpio, while the S3C SDIO
driver goes 25MHz and has a 4 bit bus to start with.

So it seems that my GTA02 isn't quite happy with the WLAN in general.
Might be because all its shields are off ...

Next: try the accelerated SPI driver.

- Werner

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