spi: introduce fallback to pio
Add fallback to pio mode in case dma transfer failed with error status SPI_TRANS_FAIL_NO_START. If spi client driver want to enable this feature please set xfer->error in the proper place such as dmaengine_prep_slave_sg() failure detect(but no any data put into spi bus yet). Besides, add master->fallback checking in its can_dma() so that spi core could switch to pio next time. Please refer to spi-imx.c. Signed-off-by: Robin Gong <yibin.gong@nxp.com> Link: https://lore.kernel.org/r/1592347329-28363-2-git-send-email-yibin.gong@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>
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