linux-kernel - omap2-mcspi, invalid SPI data received

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Date:	Thu, 18 Aug 2016 13:47:56 +0200
From:	Jonatan Magnusson <jonatan.magnusson@...il.com>
To:	broonie@...nel.org, linux-spi@...r.kernel.org,
	linux-kernel@...r.kernel.org, samuel.ortiz@...ia.com,
	juha.yrjola@...ia.com
Subject: omap2-mcspi, invalid SPI data received

Hi,

I have this problem that originally started when I tried to use an ENC28J60 device on a Beaglebone (Beaglebone Green Wireless). I noted that while reading one register on the device (EREVID) the value was 0x3C. That register holds the hardware revision and should not be higher than 6 or possibly 7. So I started investigating the code and to make a long story short I ended up writing this silly loop that continuously reads and prints out EREVID:

for(;;) {
  printk(KERN_ALERT "REV: 0x%02x\n", nolock_regb_read(priv, EREVID));
}

It reports 0x3C every time so it’s not completely random. If I force the pin to 0V i get 0x00 instead, and 0xFF if I force the pin to 3.3V. That should mean that the hardware config and pinmux is correct, since otherwise it would not change anything if I manipulate the pin.

With this loop active I could investigate what happens with a scope and to my surprise both the sent byte and the received byte are what to be expected: 0x12 and 0x06.  But somehow the SPI driver still reads 0x3C instead of 0x06.

Here’s two screenshots from the scope: https://dl.dropboxusercontent.com/u/176615/erevid.png

The first shows clock and mosi, the second clock and miso.

I’ve tried to track the error and got as far as drivers/spi/spi-omap2-mcspi.c, function omap2_mcspi_txrx_pio. Around line 715, readl_relaxed returns 0x3C.

Any ideas?

Best regards,
Jonatan Magnusson