spi: dw: Fix Rx-only DMA transfers

Tx-only DMA transfers are working perfectly fine since in this case
the code just ignores the Rx FIFO overflow interrupts. But it turns
out the SPI Rx-only transfers are broken since nothing pushing any
data to the shift registers, so the Rx FIFO is left empty and the
SPI core subsystems just returns a timeout error. Since DW DMAC
driver doesn't support something like cyclic write operations of
a single byte to a device register, the only way to support the
Rx-only SPI transfers is to fake it by using a dummy Tx-buffer.
This is what we intend to fix in this commit by setting the
SPI_CONTROLLER_MUST_TX flag for DMA-capable platform.

Signed-off-by: Serge Semin <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Cc: Georgy Vlasov <[email protected]>
Cc: Ramil Zaripov <[email protected]>
Cc: Alexey Malahov <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Feng Tang <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>

drivers/spi/spi-dw.c

@@ -515,6 +515,7 @@ int dw_spi_add_host(struct device *dev, struct dw_spi *dws)
 			dev_warn(dev, "DMA init failed\n");
 		} else {
 			master->can_dma = dws->dma_ops->can_dma;
+			master->flags |= SPI_CONTROLLER_MUST_TX;
 		}
 	}