spi: omap2-mcspi: add support for interword delay

The module omap2-mcspi does not support the interword delay
parameter present in the spi transfer. On one side, if the module
is instructed to use the dma, this parameter is correctly ignored.
However, without the usage of the dma, that parameter should be
used.

The patch introduce the handling of such delay in the omap2-mcspi
module, using standard spi_delay struct. The patch has been tested
using as benchmark a DM3730.

The delay function used (spi_delay_exec) is already present in the
kernel and it checks on its own the validity of the input, as such,
no additional checks are present.

The range of usage of the udelay function is incremented to 200 us,
as the change from udelay to usleep_range introduces not
neglectible delays.

Signed-off-by: Andrea Zanotti <andreazanottifo@gmail.com>
Link: https://lore.kernel.org/r/20220502111300.24754-1-andreazanottifo@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-omap2-mcspi.c

@@ -757,6 +757,8 @@ omap2_mcspi_txrx_pio(struct spi_device *spi, struct spi_transfer *xfer)
 				dev_vdbg(&spi->dev, "read-%d %02x\n",
 						word_len, *(rx - 1));
 			}
+			/* Add word delay between each word */
+			spi_delay_exec(&xfer->word_delay, xfer);
 		} while (c);
 	} else if (word_len <= 16) {
 		u16		*rx;
@@ -804,6 +806,8 @@ omap2_mcspi_txrx_pio(struct spi_device *spi, struct spi_transfer *xfer)
 				dev_vdbg(&spi->dev, "read-%d %04x\n",
 						word_len, *(rx - 1));
 			}
+			/* Add word delay between each word */
+			spi_delay_exec(&xfer->word_delay, xfer);
 		} while (c >= 2);
 	} else if (word_len <= 32) {
 		u32		*rx;
@@ -851,6 +855,8 @@ omap2_mcspi_txrx_pio(struct spi_device *spi, struct spi_transfer *xfer)
 				dev_vdbg(&spi->dev, "read-%d %08x\n",
 						word_len, *(rx - 1));
 			}
+			/* Add word delay between each word */
+			spi_delay_exec(&xfer->word_delay, xfer);
 		} while (c >= 4);
 	}