spi: tests driver to test basic alignment/length issues via loopback

[Martin Sperl <kernel@xxxxxxxxxxxxxxxx>]

Hi!

While writing a patch that allows spi_masters (and maybe also the
framework) to modify a submitted spi_message so that the resulting
spi_message fits the “limitations” of the HW (this basically works
already, but I still need to split it into several patches and run
further testing before submitting...) I had to generate a test
harness to make sure that all situations were taken care of properly.

So I had to write a small test driver that allows for generate all sorts
of spi_messages with different length, rx/tx_buf alignments and more to
get generated - right now it submits something like 360 distinct
spi_messages:
* transfers starting on a page with rx and tx_buf set
  * length: 1, 16, 32, 64, 128, 256, 1024, 4096, 128k
  * alignment of rx_buf/tx_buf to 0, 1, 2, 3 ([0:sizeof(int)[)
    (if your spi_master defines dma_alignment, then [0:dma_alignment[)
  * a total of 144 spi-messages
* transfer starting on (page_size - 4) with rx and tx_buf set
  * same as above
  * a total of 144 spi-messages
* tx-only transfers
  * same as above, but only alignment for tx
  * total of 36 spi-messages
* rx-only transfers
  * same as above, but only alignment for rx
  * total of 36 spi-messages

Currently no multi-transfers are tested, but in principle these can 
get defined very easily.

It is quite extensible and has already helped me identify some regressions
that the “modify” framework was introducing on the raspberry Pi.

For complete testing the only thing it requires is  MOSI connected to MISO.
and then it can detect lots of situations:
* incomplete writes in rx_buf
* writes outside of spi_transfer.rx_buf (dma writing to much)
* alignment issues

There is also several distinct tx-payload generators that can get used to
identify some issues - it may also help when using a logic-analyzer to see
the data that is really sent out.

Obviously it is not perfect and is only tested using 4 wire mode, but it
may act as a “tool” to test for some basic regressions.

So I wonder if there would be some interest in having such a test driver
in the kernel for use during spi_master development.

If interested, you can have a look at: https://github.com/msperl/spi-test <https://github.com/msperl/spi-test>
and if that seems to fulfill a requirement, then I can submit it as a
patch.

Thanks,
	Martin--
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