Re: [RFC PATCH] dmaengine: xilinx_dma: Fix per-channel direction reporting via device_caps

[Folker Schwesinger]

On Wed Mar 4, 2026 at 9:35 AM CET, Rahul Navale wrote:
>
> <4>[    0.302360] dma_slave_caps:
> <4>[    0.302364]   src_addr_widths    = 0x00000000
> <4>[    0.302368]   dst_addr_widths    = 0x00000000
> <4>[    0.302371]   directions         = 0x00000000
> <4>[    0.302374]   min_burst          = 0x00000000
> <4>[    0.302377]   max_burst          = 0x00000000
> <4>[    0.302380]   max_sg_burst       = 0x00000000
> <4>[    0.302383]   cmd_pause          = 0x00
> <4>[    0.302386]   cmd_resume         = 0x00
> <4>[    0.302388]   cmd_terminate      = 0x00
> <4>[    0.302391]   residue_granularity= 0x00000000
> <4>[    0.302394]   descriptor_reuse   = 0x00
> <4>[    0.302398] xilinx_dma_device_caps: caps->directions = 0x00000001
> <4>[    0.302401] xilinx_dma_device_caps: caps->directions = 0x00000001
> <4>[    0.302404] dma_slave_caps:
> <4>[    0.302406]   src_addr_widths    = 0x00000000
> <4>[    0.302409]   dst_addr_widths    = 0x00000000
> <4>[    0.302412]   directions         = 0x00000001
> <4>[    0.302415]   min_burst          = 0x00000000
> <4>[    0.302418]   max_burst          = 0x00000000
> <4>[    0.302421]   max_sg_burst       = 0x00000000
> <4>[    0.302423]   cmd_pause          = 0x00
> <4>[    0.302426]   cmd_resume         = 0x00
> <4>[    0.302429]   cmd_terminate      = 0x01
> <4>[    0.302431]   residue_granularity= 0x00000001
> <4>[    0.302434]   descriptor_reuse   = 0x00

I think the issue stems from the difference in residue_granularity
(directions is set in xilinx_dma_device_caps and playback is still fixed
and cmd_terminate is only referenced in the code by some unrelated
driver).

>From a quick trace through the code I found that in the Xilinx DMA
residue_granularity is set for AXIDMA independently from the SG setting
of the DMA core [1].
However, in xilinx_dma_tx_status() one of the conditions for residue
calculations is that SG mode is enabled [2].

Here's a quick patch that defers setting residue_granularity for the
device into channel probe until chan->has_sg is available. Could you
test if this fixes your issue (and of course re-activate all the caps->
assignments in dma_get_slave_caps(), keep the debug stuff for now)?

<-->8-->

diff --git i/drivers/dma/xilinx/xilinx_dma.c w/drivers/dma/xilinx/xilinx_dma.c
index fabff602065f..adae1b37b533 100644
--- i/drivers/dma/xilinx/xilinx_dma.c
+++ w/drivers/dma/xilinx/xilinx_dma.c
@@ -3039,6 +3039,14 @@ static int xilinx_dma_chan_probe(struct xilinx_dma_device *xdev,
 			str_enabled_disabled(chan->has_sg));
 	}

+	/* Residue calculation is supported by only AXI DMA and CDMA */
+	if(chan->has_sg && (
+	   xdev->dma_config->dmatype == XDMA_TYPE_AXIDMA ||
+	   xdev->dma_config->dmatype == XDMA_TYPE_CDMA)) {
+		xdev->common.residue_granularity =
+					  DMA_RESIDUE_GRANULARITY_SEGMENT;
+	}
+
 	/* Initialize the tasklet */
 	tasklet_setup(&chan->tasklet, xilinx_dma_do_tasklet);

@@ -3277,15 +3285,9 @@ static int xilinx_dma_probe(struct platform_device *pdev)
 		xdev->common.device_prep_slave_sg = xilinx_dma_prep_slave_sg;
 		xdev->common.device_prep_dma_cyclic =
 					  xilinx_dma_prep_dma_cyclic;
-		/* Residue calculation is supported by only AXI DMA and CDMA */
-		xdev->common.residue_granularity =
-					  DMA_RESIDUE_GRANULARITY_SEGMENT;
 	} else if (xdev->dma_config->dmatype == XDMA_TYPE_CDMA) {
 		dma_cap_set(DMA_MEMCPY, xdev->common.cap_mask);
 		xdev->common.device_prep_dma_memcpy = xilinx_cdma_prep_memcpy;
-		/* Residue calculation is supported by only AXI DMA and CDMA */
-		xdev->common.residue_granularity =
-					  DMA_RESIDUE_GRANULARITY_SEGMENT;
 	} else if (xdev->dma_config->dmatype == XDMA_TYPE_AXIMCDMA) {
 		xdev->common.device_prep_slave_sg = xilinx_mcdma_prep_slave_sg;
 	} else {

<--8<-->

[1]: https://elixir.bootlin.com/linux/v6.19.3/source/drivers/dma/xilinx/xilinx_dma.c#L3284
[2]: https://elixir.bootlin.com/linux/v6.19.3/source/drivers/dma/xilinx/xilinx_dma.c#L1293