Hello,
I just encountered a problem on my ARMv5 (AT91SAM9263 based) board while using
ads7843 or ads7846 (I replaced it). I tried reading
/sys/class/hwmon/hwmon0/device/in0_input and got 0 most of the time. Sometimes
the correct voltage was shown.
After digging a bit in the code I found out that sample (and command for Tx)
from the following struct gets DMA mapped by the underlying atmel_spi driver
for SPI Rx-Buffer.
struct ser_req {
u8 ref_on;
u8 command;
u8 ref_off;
u16 scratch;
__be16 sample;
struct spi_message msg;
struct spi_transfer xfer[6];
};
The atmel_spi driver does proper dma_{,un}map_signgle on the Rx and Tx
buffers. Now when the cache line for scratch is cleared and invalidated and
the spi subsystem access msg (and maybe xfer) the cache line gets filled
again, rendering the cache clear and invalidation before as useless.
As the ARMv5 core has 32Byte cache lines I added "char dummy[25];" after
sample, so the spi structs are in a different cache line.
But this seem only to be a workaround as the cache lines might differ across
each CPU.
Somebody an idea how to fix this in general?
Regards,
Alexander
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