[PATCH] spi: mediatek: Manually set dma_ops for spi_master device

[Daniel Kurtz]

Back before commit 1dccb598df54 ("arm64: simplify dma_get_ops"), for arm64,
devices that do not manually set a dma_ops are automatically configured to
use swiotlb_dma_ops, since this was hard-coded as the global "dma_ops" in
arm64_dma_init().

Now, the global "dma_ops" has been removed, and all devices much have
their dma_ops explicitly set by a call to arch_setup_dma_ops().  If
arch_setup_dma_ops() is not called, the device is assigned dummy_dma_ops,
and thus calls to map_sg for such a device will fail (return 0).

Mediatek SPI uses DMA but does not use a dma channel.  Support for this
was added by commit c37f45b5f1cd ("spi: support spi without dma channel to
use can_dma()"), which uses the master_spi dev to DMA map buffers.

The master_spi device is not a platform, rather it is created
in spi_alloc_device(), and its dma_ops are never set.

Therefore, when the mediatek SPI driver when it does DMA (for large SPI
transactions > 32 bytes), SPI will use spi_map_buf()->dma_map_sg() to map
the buffer for use in DMA.  But dma_map_sg()->dma_map_sg_attrs() returns 0,
because ops->map_sg is dummy_dma_ops->__dummy_map_sg, and hence
spi_map_buf() returns -ENOMEM (-12).

The solution in this patch is a bit of a hack.  To install dma_ops for
its spi_master, we call of_dma_configure() during probe and pass in the
of_node of the spi-mt65xx platform device.

However, by default, of_dma_configure() will set the coherent_dma_mask
to 0xffffffff.  In fact, using a non-zero dma_mask doesn't actually work
and causes frequent SPI transaction errors.  To work around this, we
also explicitly set coherent_dma_mask to 0.

Signed-off-by: Daniel Kurtz <djkurtz at chromium.org>
---

I don't know the right place to configure the dma_ops for spi_master.
It feels like this should actually be done in the spi core, as this might be
needed by other drivers.

Alternatively, perhaps we should be using master->dev.parent to dma map bufs
in the "no dma channel case".

And I really don't know why we needed to set the coherent_dma_mask to 0 to
avoid SPI transaction errors.

Any advice is welcome.

 drivers/spi/spi-mt65xx.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/spi/spi-mt65xx.c b/drivers/spi/spi-mt65xx.c
index 8763eff13d3d..e768835bb67f 100644
--- a/drivers/spi/spi-mt65xx.c
+++ b/drivers/spi/spi-mt65xx.c
@@ -20,6 +20,7 @@
 #include <linux/ioport.h>
 #include <linux/module.h>
 #include <linux/of.h>
+#include <linux/of_device.h>
 #include <linux/of_gpio.h>
 #include <linux/platform_device.h>
 #include <linux/platform_data/spi-mt65xx.h>
@@ -575,6 +576,10 @@ static int mtk_spi_probe(struct platform_device *pdev)
 		goto err_put_master;
 	}
 
+	/* Call of_dma_configure to set up spi_master's dma_ops */
+	of_dma_configure(&master->dev, master->dev.of_node);
+	/* But explicitly set the coherent_dma_mask to 0 */
+	master->dev.coherent_dma_mask = 0;
 	if (!pdev->dev.dma_mask)
 		pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
 
-- 
2.11.0.483.g087da7b7c-goog