[RFC PATCH] mfd: mfd-core: inherit only valid dma_masks/flags from parent

[Michael Walle <michael@walle.cc>]

Only copy the dma_masks and flags from the parent device, if the parent
has a valid dma_mask/flags. Commit cdfee5623290 ("driver core:
initialize a default DMA mask for platform device") initialize the DMA
masks of a platform device. But if the parent doesn't have a dma_mask
set, for example if it's an I2C device, the dma_mask of the child
platform device will be set to zero again. Which leads to many "DMA mask
not set" warnings, if the MFD cell has the of_compatible property set.

[    1.877937] sl28cpld-pwm sl28cpld-pwm: DMA mask not set
[    1.883282] sl28cpld-pwm sl28cpld-pwm.0: DMA mask not set
[    1.888795] sl28cpld-gpio sl28cpld-gpio: DMA mask not set

Thus a MFD child should just inherit valid dma_masks and keep the
platform default otherwise.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Rob Herring <robh@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Michael Walle <michael@walle.cc>
---

Hi,

I don't know if that is the correct way of handling things. Maybe I'm
also doing something wrong in my driver, I had a look at other I2C MFD
drivers but couldn't find a clue why they shouldn't have the same
problem.

There was also a thread [1] about this topic, but there seems to be no
conclusion.

[1] https://www.spinics.net/lists/linux-renesas-soc/msg31581.html

 drivers/mfd/mfd-core.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/mfd/mfd-core.c b/drivers/mfd/mfd-core.c
index b9eb8f40c073..5d8ea5e8e93c 100644
--- a/drivers/mfd/mfd-core.c
+++ b/drivers/mfd/mfd-core.c
@@ -139,9 +139,12 @@ static int mfd_add_device(struct device *parent, int id,
 
 	pdev->dev.parent = parent;
 	pdev->dev.type = &mfd_dev_type;
-	pdev->dev.dma_mask = parent->dma_mask;
-	pdev->dev.dma_parms = parent->dma_parms;
-	pdev->dev.coherent_dma_mask = parent->coherent_dma_mask;
+	if (parent->dma_mask)
+		pdev->dev.dma_mask = parent->dma_mask;
+	if (parent->dma_parms)
+		pdev->dev.dma_parms = parent->dma_parms;
+	if (parent->coherent_dma_mask)
+		pdev->dev.coherent_dma_mask = parent->coherent_dma_mask;
 
 	ret = regulator_bulk_register_supply_alias(
 			&pdev->dev, cell->parent_supplies,
-- 
2.20.1