[PATCH AUTOSEL 6.6 6/8] dmaengine: xilinx_dma: Set dma_device directions

[PATCH AUTOSEL 6.6 6/8] dmaengine: xilinx_dma: Set dma_device directions

[Sasha Levin]

From: Thomas Gessler <thomas.gessler@brueckmann-gmbh.de>

[ Upstream commit 7e01511443c30a55a5ae78d3debd46d4d872517e ]

Coalesce the direction bits from the enabled TX and/or RX channels into
the directions bit mask of dma_device. Without this mask set,
dma_get_slave_caps() in the DMAEngine fails, which prevents the driver
from being used with an IIO DMAEngine buffer.

Signed-off-by: Thomas Gessler <thomas.gessler@brueckmann-gmbh.de>
Reviewed-by: Suraj Gupta <suraj.gupta2@amd.com>
Tested-by: Folker Schwesinger <dev@folker-schwesinger.de>
Link: https://lore.kernel.org/r/20250507182101.909010-1-thomas.gessler@brueckmann-gmbh.de
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

**YES**

## Analysis

This commit should be backported to stable kernel trees because it fixes
a critical functionality bug that prevents the Xilinx DMA driver from
working with important subsystems.

### The Problem
The commit addresses a fundamental issue where `dma_get_slave_caps()`
fails when called on Xilinx DMA channels. The code change shows that the
driver was not setting the `directions` field in the `dma_device`
structure:

```c
+       xdev->common.directions |= chan->direction;
```

This single line fix accumulates the direction capabilities from
individual channels (either `DMA_MEM_TO_DEV` or `DMA_DEV_TO_MEM`) into
the device-level `directions` bitmask.

### Why This Qualifies for Backporting

1. **Fixes Important User-Facing Bug**: The commit message explicitly
   states this prevents the driver from being used with IIO DMAEngine
   buffers, which is a significant functional regression affecting real
   users.

2. **Small and Contained Fix**: The change is minimal - just one line of
   code that sets a required field during channel probe. This has
   extremely low risk of introducing regressions.

3. **Critical Subsystem Integration**: Without this fix,
   `dma_get_slave_caps()` calls fail with `-ENXIO`, breaking integration
   with any subsystem that queries DMA capabilities (like IIO).

4. **Clear Root Cause**: The fix directly addresses the root cause - the
   missing `directions` field that the DMAEngine core requires to be
   set.

5. **No Architectural Changes**: This doesn't introduce new features or
   change driver architecture; it simply provides required capability
   information that was missing.

### Comparison to Reference Commits
This closely matches **Similar Commit #1** (marked YES) which also fixed
a missing capability flag (`DMA_CYCLIC cap_mask bit`) that prevented
proper DMA channel allocation. Both commits:
- Fix missing capability declarations
- Are small, single-line changes
- Address integration failures with other subsystems
- Have minimal regression risk

The commit also mirrors **Similar Commit #2** (marked YES) which fixed
incorrect struct usage in the same driver - both address functional
correctness issues in the Xilinx DMA driver.

### Risk Assessment
The risk is minimal because:
- The change only affects the capability reporting mechanism
- It doesn't modify any data paths or transfer logic
- The direction values being OR'd together are already correctly set
  per-channel
- Failure mode is obvious (capability queries will work instead of
  failing)

This is a textbook example of a stable tree candidate: it fixes an
important bug affecting real users with a minimal, low-risk change that
doesn't introduce new functionality.

 drivers/dma/xilinx/xilinx_dma.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
index 0a3b2e22f23db..14c4c5031b556 100644
--- a/drivers/dma/xilinx/xilinx_dma.c
+++ b/drivers/dma/xilinx/xilinx_dma.c
@@ -2900,6 +2900,8 @@ static int xilinx_dma_chan_probe(struct xilinx_dma_device *xdev,
 		return -EINVAL;
 	}
 
+	xdev->common.directions |= chan->direction;
+
 	/* Request the interrupt */
 	chan->irq = of_irq_get(node, chan->tdest);
 	if (chan->irq < 0)
-- 
2.39.5