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 <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru>
Cc: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20200529131205.31838-9-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --git a/drivers/spi/spi-dw.c b/drivers/spi/spi-dw.c
index 73af58d..1f1a353 100644 (file)
--- a/drivers/spi/spi-dw.c
+++ b/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;
                }
        }