RE: [PATCH v1 RFC 1/2] spi: introduce fallback to pio

[Robin Gong <yibin.gong@nxp.com>]

On 2020/06/15 22:56 Mark Brown <broonie@kernel.org> wrote: 
> On Mon, Jun 15, 2020 at 02:53:29PM +0000, Robin Gong wrote: 
> > Do you mean spi-imx.c checking 'ctlr->flags' before return such error code?
> > Or just like below done in spi.c.
> 
> No, I mean passing in an additional argument which can provide richer data
> than trying to smash things into the return value.
Okay, how about adding this additional argument in struct spi_transfer like below?
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 95291fe4..7c19099 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -943,6 +943,9 @@ struct spi_transfer {
        bool            timestamped;

        struct list_head transfer_list;
+
+#define        SPI_TRANS_DMA_PREP_FAIL BIT(3)  /* prepare dma failed */
+       u16             flags;
 };

While spi core checking it as below after it set in spi-imx.c:
fallback_pio:
        ret = ctlr->transfer_one(ctlr, msg->spi, xfer);
        if (ret < 0) {
                 if (ctlr->cur_msg_mapped &&
                   (xfer->flags & SPI_TRANS_DMA_PREP_FAIL)) {
                   __spi_unmap_msg(ctlr, msg);
                   ctlr->fallback = true;
                   xfer->flags &= ~SPI_TRANS_DMA_PREP_FAIL;
                   goto fallback_pio;
                 }