[PATCH v7 01/10] dmaengine: Allow drivers to assign static channel IDs

[Koichiro Den]

The dmaengine core assigns channel IDs in registration order. If a driver
skips a hardware channel, chan_id can differ from the hardware numbering
and a client cannot reliably correlate a requested channel with hardware
resources.

Let a driver request an exact channel ID before device registration.
Reserve static IDs through the existing IDA so they remain unique, while
retaining automatic IDA allocation as the default.

Use direction-flattened IDs for dw-edma channels. Unlike the
direction-local hardware channel number, these IDs are unique within the
DMA device.

Suggested-by: Frank Li <Frank.Li@xxxxxxx>
Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
---
Changes in v7:
  - New patch. (Frank)
    https://lore.kernel.org/r/lm7tadnxsyrypu4mypptlkx5qkytex4qxsijdr5ydud2n3anvf@yakjfjg5ng4u/

 drivers/dma/dmaengine.c            | 13 ++++++++-----
 drivers/dma/dw-edma/dw-edma-core.c |  1 +
 include/linux/dmaengine.h          | 20 ++++++++++++++++++++
 3 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 6ffd8bd82154..cc64a4679e6f 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -1078,6 +1078,7 @@ static int __dma_async_device_channel_register(struct dma_device *device,
 					       struct dma_chan *chan,
 					       const char *name)
 {
+	unsigned int id;
 	int rc;
 
 	chan->local = alloc_percpu(typeof(*chan->local));
@@ -1089,11 +1090,13 @@ static int __dma_async_device_channel_register(struct dma_device *device,
 		goto err_free_local;
 	}
 
-	/*
-	 * When the chan_id is a negative value, we are dynamically adding
-	 * the channel. Otherwise we are static enumerating.
-	 */
-	chan->chan_id = ida_alloc(&device->chan_ida, GFP_KERNEL);
+	if (chan->chan_id & DMA_CHAN_ID_STATIC) {
+		id = chan->chan_id & ~DMA_CHAN_ID_STATIC;
+		chan->chan_id = ida_alloc_range(&device->chan_ida, id, id,
+						GFP_KERNEL);
+	} else {
+		chan->chan_id = ida_alloc(&device->chan_ida, GFP_KERNEL);
+	}
 	if (chan->chan_id < 0) {
 		pr_err("%s: unable to alloc ida for chan: %d\n",
 		       __func__, chan->chan_id);
diff --git a/drivers/dma/dw-edma/dw-edma-core.c b/drivers/dma/dw-edma/dw-edma-core.c
index 1f893dc54c79..d214df55da3c 100644
--- a/drivers/dma/dw-edma/dw-edma-core.c
+++ b/drivers/dma/dw-edma/dw-edma-core.c
@@ -987,6 +987,7 @@ static int dw_edma_channel_setup(struct dw_edma *dw, u32 wr_alloc, u32 rd_alloc)
 					&dw->chip->dt_region_rd[chan->id];
 
 		vchan_init(&chan->vc, dma);
+		dmaengine_set_static_chan_id(&chan->vc.chan, i);
 
 		dw_edma_core_ch_config(chan);
 	}
diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index fe33a20abc61..f669b79d7731 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -369,6 +369,26 @@ struct dma_chan {
 	void *private;
 };
 
+#define DMA_CHAN_ID_STATIC	BIT(30)
+
+/**
+ * dmaengine_set_static_chan_id - request an exact DMA engine channel ID
+ * @chan: DMA channel
+ * @id: channel ID, unique within the DMA device
+ *
+ * Drivers may call this after initializing @chan and before registering its
+ * DMA device. The dmaengine core reserves @id from the device IDA instead of
+ * assigning the next available ID.
+ */
+static inline void dmaengine_set_static_chan_id(struct dma_chan *chan,
+						unsigned int id)
+{
+	if (WARN_ON_ONCE(id >= DMA_CHAN_ID_STATIC))
+		return;
+
+	chan->chan_id = DMA_CHAN_ID_STATIC | id;
+}
+
 /**
  * struct dma_chan_dev - relate sysfs device node to backing channel device
  * @chan: driver channel device
-- 
2.51.0