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Vladimir Oltean authored
dspi->words_in_flight is a variable populated in the *_write functions and used in the dspi_fifo_read function. It is also used in dspi_fifo_write, immediately after transmission, to update the message->actual_length variable used by higher layers such as spi-mem for integrity checking. But it may happen that the IRQ which calls dspi_fifo_read to be triggered before the updating of message->actual_length takes place. In that case, dspi_fifo_read will decrement dspi->words_in_flight to -1, and that will cause an invalid modification of message->actual_length. For that, we make the simplest fix possible: to not decrement the actual shared variable in dspi->words_in_flight from dspi_fifo_read, but actually a copy of it which is on stack. But even if dspi_fifo_read from the next IRQ does not interfere with the dspi_fifo_write of the current chunk, the *next* dspi_fifo_write still can. So we must assume that everything after the last write to the TX FIFO can be preempted by the "TX complete" IRQ, and the dspi_fifo_write function must be safe against that. This means refactoring the 2 flavours of FIFO writes (for EOQ and XSPI) such that the calculation of the number of words to be written is common and happens a priori. This way, the code for updating the message->actual_length variable works with a copy and not with the volatile dspi->words_in_flight. After some interior debate, the dspi->progress variable used for software timestamping was *not* backed up against preemption in a copy on stack. Because if preemption does occur between spi_take_timestamp_pre and spi_take_timestamp_post, there's really no point in trying to save anything. The first-in-time spi_take_timestamp_post call with a dspi->progress higher than the requested xfer->ptp_sts_word_post will trigger xfer->timestamped = true anyway and will close the deal. To understand the above a bit better, consider a transfer with xfer->ptp_sts_word_pre = xfer->ptp_sts_word_post = 3, and xfer->bits_per_words = 8 (so byte 3 needs to be timestamped). The DSPI controller timestamps in chunks of 4 bytes at a time, and preemption occurs in the middle of timestamping the first chunk: spi_take_timestamp_pre(0) . . (preemption) . . spi_take_timestamp_pre(4) . . spi_take_timestamp_post(7) . spi_take_timestamp_post(3) So the reason I'm not bothering to back up dspi->progress for that spi_take_timestamp_post(3) is that spi_take_timestamp_post(7) is going to (a) be more honest, (b) provide better accuracy and (c) already render the spi_take_timestamp_post(3) into a noop by setting xfer->timestamped = true anyway. Fixes: d59c90a2 ("spi: spi-fsl-dspi: Convert TCFQ users to XSPI FIFO mode") Reported-by: Michael Walle <michael@walle.cc> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Tested-by: Michael Walle <michael@walle.cc> Link: https://lore.kernel.org/r/20200318001603.9650-6-olteanv@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>
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