spi-rspi I/O errors

On Fri, Jan 10, 2014 at 10:20 AM, Geert Uytterhoeven
[off-list ref] wrote:

On Thu, Jan 9, 2014 at 7:14 PM, Laurent Pinchart
[off-list ref] wrote:

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It seems to get a stale value. I ended up doing the printk() anyway, and
when reading using small buffer sizes, the issues is more likely to
happen.

If I add a second copy of rspi->spsr, and make that copy volatile OR add
a few calls to smp_mb() around the places where it's written and read,
the copy has the right value inside rspi_wait_for_interrupt(), BUT only if
I also print the spsr value inside rspi_irq(). Note that the original
rspi->spsr still has the stale value.

Do I understand it correctly that the SPSR value read from the register in
rspi_irq() is always correct on the first read, and that only the rspi->spsr
value stored in memory and read in rspi_wait_for_interrupt() is wrong ?

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I'd expect wake_up() and wait_event_timeout() to have the right memory
barriers (cfr. Documentation/memory-barriers.txt), so I don't have to add
my own. And why does it need the extra printk()? What extra
synchronization
does that give?

Have you tried adding explicit memory barriers without any extra printk ?

Yes, cfr. "add a few calls to smp_mb()" above.

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Note that the interrupt handler always runs on CPU core 0, while
rspi_wait_for_interrupt() can run on core 0 or 1. Is there a
cache-coherency issue between the two CPU cores?

I'd be very surprised if that was the case, as there should be lots of other
breakages, but let's not rule that out too fast. Magnus should be able to
help you more than I can regarding that.

Could you verify that the SMP bit is set in ACTLR
(http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0388e/CIHHDDHJ.html)

ACTLR = 0x000000c0 on CPU 0
ACTLR = 0x00000040 on CPU 1

0xc0 = EXCL | SMP
0x40 = SMP

The different EXCL value seems strange to me, especially considering the comment
about the matching cache controller configuration:

I tried enabling the EXCL bit for ACTLR on CPU 1, but it doesn't make
a difference.

One other thing: As I was playing with m25p80, I had #define DEBUG at the
top of drivers/mtd/devices/m25p80.c. This doesn't cause much kernel output,
just one message before every read:

    spi0.0: m25p80_read from 0x00108600, len 512

However, without the #define DEBUG, it's _much_ more difficult to trigger
the issue.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds