net: netsec: reduce DMA mask to 40 bits

The netsec network controller IP can drive 64 address bits for DMA, and
the DMA mask is set accordingly in the driver. However, the SynQuacer
SoC, which is the only silicon incorporating this IP at the moment,
integrates this IP in a manner that leaves address bits [63:40]
unconnected.

Up until now, this has not resulted in any problems, given that the DDR
controller doesn't decode those bits to begin with. However, recent
firmware updates for platforms incorporating this SoC allow the IOMMU
to be enabled, which does decode address bits [47:40], and allocates
top down from the IOVA space, producing DMA addresses that have bits
set that have been left unconnected.

Both the DT and ACPI (IORT) descriptions of the platform take this into
account, and only describe a DMA address space of 40 bits (using either
dma-ranges DT properties, or DMA address limits in IORT named component
nodes). However, even though our IOMMU and bus layers may take such
limitations into account by setting a narrower DMA mask when creating
the platform device, the netsec probe() entrypoint follows the common
practice of setting the DMA mask uncondionally, according to the
capabilities of the IP block itself rather than to its integration into
the chip.

It is currently unclear what the correct fix is here. We could hack around
it by only setting the DMA mask if it deviates from its default value of
DMA_BIT_MASK(32). However, this makes it impossible for the bus layer to
use DMA_BIT_MASK(32) as the bus limit, and so it appears that a more
comprehensive approach is required to take DMA limits imposed by the
SoC as a whole into account.

In the mean time, let's limit the DMA mask to 40 bits. Given that there
is currently only one SoC that incorporates this IP, this is a reasonable
approach that can be backported to -stable and buys us some time to come
up with a proper fix going forward.

Fixes: 533dd11a12 ("net: socionext: Add Synquacer NetSec driver")
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Jassi Brar <jaswinder.singh@linaro.org>
Cc: Masahisa Kojima <masahisa.kojima@linaro.org>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/socionext/netsec.c

@@ -1674,8 +1674,8 @@ static int netsec_probe(struct platform_device *pdev)
 	if (ret)
 		goto unreg_napi;
 
-	if (dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64)))
+	if (dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(40)))
-		dev_warn(&pdev->dev, "Failed to enable 64-bit DMA\n");
+		dev_warn(&pdev->dev, "Failed to set DMA mask\n");
 
 	ret = register_netdev(ndev);
 	if (ret) {