Remote DMA users may need to map or otherwise correlate DMA resources on
a per-hardware-channel basis (e.g. DWC EP eDMA linked-list windows).
However, struct dma_chan does not expose a provider-defined hardware
channel identifier.

Add an optional dma_slave_caps.hw_id field to allow DMA engine drivers
to report a provider-specific hardware channel identifier to clients.
Initialize the field to -1 in dma_get_slave_caps() so drivers that do
not populate it continue to behave as before.

Signed-off-by: Koichiro Den <redacted>
---
 drivers/dma/dmaengine.c   | 1 +
 include/linux/dmaengine.h | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index ca13cd39330b..b544eb99359d 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c

@@ -603,6 +603,7 @@ int dma_get_slave_caps(struct dma_chan *chan, struct dma_slave_caps *caps)
 	caps->cmd_pause = !!device->device_pause;
 	caps->cmd_resume = !!device->device_resume;
 	caps->cmd_terminate = !!device->device_terminate_all;
+	caps->hw_id = -1;

 	/*
 	 * DMA engine device might be configured with non-uniformly

diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 99efe2b9b4ea..71bc2674567f 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h

@@ -507,6 +507,7 @@ enum dma_residue_granularity {
  * @residue_granularity: granularity of the reported transfer residue
  * @descriptor_reuse: if a descriptor can be reused by client and
  * resubmitted multiple times
+ * @hw_id: provider-specific hardware channel identifier (-1 if unknown)
  */
 struct dma_slave_caps {
 	u32 src_addr_widths;

@@ -520,6 +521,7 @@ struct dma_slave_caps {
 	bool cmd_terminate;
 	enum dma_residue_granularity residue_granularity;
 	bool descriptor_reuse;
+	int hw_id;