[PATCH v3] of/platform: initialise AMBA default DMA masks

[Linus Walleij]

This addresses a v4.19-rc1 regression in the PL111 DRM driver
in drivers/gpu/pl111/*

The driver uses the CMA KMS helpers and will thus at some
point call down to dma_alloc_attrs() to allocate a chunk
of contigous DMA memory for the framebuffer.

It appears that in v4.18, it was OK that this (and other
DMA mastering AMBA devices) left dev->coherent_dma_mask
blank (zero).

In v4.19-rc1 the WARN_ON_ONCE(dev && !dev->coherent_dma_mask)
in dma_alloc_attrs() in include/linux/dma-mapping.h is
triggered. The allocation later fails when get_coherent_dma_mask()
is called from __dma_alloc() and __dma_alloc() returns
NULL:

drm-clcd-pl111 dev:20: coherent DMA mask is unset
drm-clcd-pl111 dev:20: [drm:drm_fb_helper_fbdev_setup] *ERROR*
 	       	        Failed to set fbdev configuration

It turns out that in commit 4d8bde883bfb
("OF: Don't set default coherent DMA mask")
the OF core stops setting the default DMA mask on new devices,
especially those lines of the patch:

- if (!dev->coherent_dma_mask)
-               dev->coherent_dma_mask = DMA_BIT_MASK(32);

Robin Murphy solved a similar problem in
a5516219b102 ("of/platform: Initialise default DMA masks")
by simply assigning dev.coherent_dma_mask and the
dev.dma_mask to point to the same when creating devices
from the device tree, and introducing the same code into
the code path creating AMBA/PrimeCell devices solved my
problem, graphics now come up.

The code simply assumes that the device can access all
of the system memory by setting the coherent DMA mask
to 0xffffffff when creating a device from the device
tree, which is crude, but seems to be what kernel v4.18
assumed.

The AMBA PrimeCells do not differ between coherent and
streaming DMA so we can just assign the same to any
DMA mask.

Possibly drivers should augment their coherent DMA mask
in accordance with "dma-ranges" from the device tree
if more finegranular masking is needed.

Reported-by: Russell King <linux at armlinux.org.uk>
Fixes: 4d8bde883bfb ("OF: Don't set default coherent DMA mask")
Cc: Russell King <linux at armlinux.org.uk>
Cc: Christoph Hellwig <hch at lst.de>
Cc: Robin Murphy <robin.murphy at arm.com>
Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
---
ChangeLog v2->v3:
- Provide proper root cause analysis, point to the right
  offending commit with Fixes:
- Make even more elaborate description of what is causing
  this.
ChangeLog v1->v2:
- Provide a better description for the change.
---
 drivers/of/platform.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/of/platform.c b/drivers/of/platform.c
index 7ba90c290a42..6c59673933e9 100644
--- a/drivers/of/platform.c
+++ b/drivers/of/platform.c
@@ -241,6 +241,10 @@ static struct amba_device *of_amba_device_create(struct device_node *node,
 	if (!dev)
 		goto err_clear_flag;
 
+	/* AMBA devices only support a single DMA mask */
+	dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
+	dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
+
 	/* setup generic device info */
 	dev->dev.of_node = of_node_get(node);
 	dev->dev.fwnode = &node->fwnode;
-- 
2.17.1