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Other interrupt sources are masked in XSPI mode and should be > > > irrelevant. > > > > Yes complete(&dspi->xfer_done) is called so CMDTCF is set. For example in > > one case of underflow I get SPI_SR = 0xca8b0450, which is these flags: > > > > TCF, TXRXS, TFUF, TFFF, CMDTCF, RFOF, RFDF, CMDFFF > > > > Compared to a successful transfer I get 0xc2830330: > > > > TCF, TXRXS, TFFF, CMDTCF, RFDF, CMDFFF > > Ok, so my new question would be: if CMDTCF is set, presumably it means a > command was transferred. What command was transferred, and who put data > in the FIFO for it? > > Because the answer to the above is AFAIU "no one", I guess the driver > should ignore CMDTCF when TFUF (TX FIFO underflow) is set; I consider > that to be the logic bug. You are also doing that in patch 4/4, except > you still call complete() for some reason. If you don't call complete(), > there is no reason to fend against spurious completions. > > I think I would prefer seeing more deliberate decisions in the driver, > it helps if things don't just work by coincidence. After thinking some more, I think I agree with your decision. If there's a TX FIFO underflow in target mode, presumably there are 2 cases to handle. 1. The underflow occurred in the middle of a large-ish SPI message prepared by the driver, where the driver couldn't refill the TX FIFO fast enough in dspi_interrupt(). 2. The underflow occurred because the driver had absolutely no SPI message prepared, and yet the host wanted something. What changed my mind is that if you don't call complete() on SPI_SR_TFUF (like I suggested), then case #1 above will hang. Your proposal is to call complete() anyway, but to discard any previous completions, associated with case #2, when there's a new message to prepare. But I would like you to introduce a comment above the earlier reinit_completion() explaining why it is there.