Re: [PATCH v2 12/19] spi: dw: Fix Rx-only DMA transfers

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

On Fri, May 15, 2020 at 01:47:51PM +0300, Serge Semin wrote:
> 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.

I'm fine with this if Mark considers this right thing to do.
So, conditionally
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

> 
> Signed-off-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
> Cc: Georgy Vlasov <Georgy.Vlasov@xxxxxxxxxxxxxxxxxxxx>
> Cc: Ramil Zaripov <Ramil.Zaripov@xxxxxxxxxxxxxxxxxxxx>
> Cc: Alexey Malahov <Alexey.Malahov@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
> Cc: Paul Burton <paulburton@xxxxxxxxxx>
> Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Allison Randal <allison@xxxxxxxxxxx>
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Cc: Gareth Williams <gareth.williams.jx@xxxxxxxxxxx>
> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
> Cc: linux-mips@xxxxxxxxxxxxxxx
> Cc: devicetree@xxxxxxxxxxxxxxx
> ---
>  drivers/spi/spi-dw.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/spi/spi-dw.c b/drivers/spi/spi-dw.c
> index 1edb8cdd11ee..31607b40147d 100644
> --- a/drivers/spi/spi-dw.c
> +++ b/drivers/spi/spi-dw.c
> @@ -517,6 +517,7 @@ 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;
>  		}
>  	}
>  
> -- 
> 2.25.1
> 

-- 
With Best Regards,
Andy Shevchenko