[PATCH v2 03/12] dmaengine: dw-edma: Add partial channel ownership mode

[Koichiro Den]

Some endpoint DMA frontends expose only a subset of a controller that is
also initialized by the endpoint-side OS. Add a partial ownership flag
so dw-edma does not reset controller-wide state in probe() or remove().

Keep the mode conservative. Do not enable interrupt-emulation doorbells,
and reject partial instances for map formats that this driver cannot safely
share. For EDMA_MF_EDMA_UNROLL and EDMA_MF_HDMA_COMPAT, require ownership
of all channels in each exposed direction. The driver updates registers
shared by all channels in a direction, such as interrupt masks and
linked-list error enables, so two independent OS instances cannot safely
split one direction without a shared locking protocol, which is
unrealistic.

The frontend must still quiesce delegated channels before removing a
partial instance. The flag only keeps probe() and remove() from
resetting controller-wide state that may belong to a peer OS instance.

Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
---
Changes in v2:
  - Reject partial ownership for unsupported map formats up front,
    keep direction-granularity validation limited to supported formats.
  - Revise the commit message accordingly.

 drivers/dma/dw-edma/dw-edma-core.c | 47 +++++++++++++++++++++++-------
 include/linux/dma/edma.h           |  6 ++++
 2 files changed, 43 insertions(+), 10 deletions(-)

diff --git a/drivers/dma/dw-edma/dw-edma-core.c b/drivers/dma/dw-edma/dw-edma-core.c
index a70e0640d082..fcef9a27b6ce 100644
--- a/drivers/dma/dw-edma/dw-edma-core.c
+++ b/drivers/dma/dw-edma/dw-edma-core.c
@@ -794,6 +794,9 @@ static int dw_edma_emul_irq_alloc(struct dw_edma *dw)
 	chip->db_irq = 0;
 	chip->db_offset = ~0;
 
+	if (chip->flags & DW_EDMA_CHIP_PARTIAL)
+		return 0;
+
 	/*
 	 * Only meaningful when the core provides the deassert sequence
 	 * for interrupt emulation.
@@ -1135,6 +1138,8 @@ int dw_edma_probe(struct dw_edma_chip *chip)
 {
 	struct device *dev;
 	struct dw_edma *dw;
+	u16 hw_wr_ch_cnt;
+	u16 hw_rd_ch_cnt;
 	u32 wr_alloc = 0;
 	u32 rd_alloc = 0;
 	int i, err;
@@ -1146,6 +1151,16 @@ int dw_edma_probe(struct dw_edma_chip *chip)
 	if (!dev || !chip->ops)
 		return -EINVAL;
 
+	if (chip->flags & DW_EDMA_CHIP_PARTIAL) {
+		switch (chip->mf) {
+		case EDMA_MF_EDMA_UNROLL:
+		case EDMA_MF_HDMA_COMPAT:
+			break;
+		default:
+			return -EOPNOTSUPP;
+		}
+	}
+
 	dw = devm_kzalloc(dev, sizeof(*dw), GFP_KERNEL);
 	if (!dw)
 		return -ENOMEM;
@@ -1159,13 +1174,23 @@ int dw_edma_probe(struct dw_edma_chip *chip)
 
 	raw_spin_lock_init(&dw->lock);
 
-	dw->wr_ch_cnt = min_t(u16, chip->ll_wr_cnt,
-			      dw_edma_core_ch_count(dw, EDMA_DIR_WRITE));
-	dw->wr_ch_cnt = min_t(u16, dw->wr_ch_cnt, EDMA_MAX_WR_CH);
+	hw_wr_ch_cnt = min_t(u16, dw_edma_core_ch_count(dw, EDMA_DIR_WRITE),
+			     EDMA_MAX_WR_CH);
+	hw_rd_ch_cnt = min_t(u16, dw_edma_core_ch_count(dw, EDMA_DIR_READ),
+			     EDMA_MAX_RD_CH);
+
+	if (chip->flags & DW_EDMA_CHIP_PARTIAL) {
+		/*
+		 * Direction-wide registers are shared by all channels in that
+		 * direction, so a direction must have a single owner.
+		 */
+		if ((chip->ll_wr_cnt && chip->ll_wr_cnt != hw_wr_ch_cnt) ||
+		    (chip->ll_rd_cnt && chip->ll_rd_cnt != hw_rd_ch_cnt))
+			return -EOPNOTSUPP;
+	}
 
-	dw->rd_ch_cnt = min_t(u16, chip->ll_rd_cnt,
-			      dw_edma_core_ch_count(dw, EDMA_DIR_READ));
-	dw->rd_ch_cnt = min_t(u16, dw->rd_ch_cnt, EDMA_MAX_RD_CH);
+	dw->wr_ch_cnt = min_t(u16, chip->ll_wr_cnt, hw_wr_ch_cnt);
+	dw->rd_ch_cnt = min_t(u16, chip->ll_rd_cnt, hw_rd_ch_cnt);
 
 	if (!dw->wr_ch_cnt && !dw->rd_ch_cnt)
 		return -EINVAL;
@@ -1182,8 +1207,10 @@ int dw_edma_probe(struct dw_edma_chip *chip)
 	snprintf(dw->name, sizeof(dw->name), "dw-edma-core:%s",
 		 dev_name(chip->dev));
 
-	/* Disable eDMA, only to establish the ideal initial conditions */
-	dw_edma_core_off(dw);
+	if (!(chip->flags & DW_EDMA_CHIP_PARTIAL)) {
+		/* Disable eDMA only when this instance owns the controller. */
+		dw_edma_core_off(dw);
+	}
 
 	/* Request IRQs */
 	err = dw_edma_irq_request(dw, &wr_alloc, &rd_alloc);
@@ -1227,8 +1254,8 @@ int dw_edma_remove(struct dw_edma_chip *chip)
 	if (!dw)
 		return -ENODEV;
 
-	/* Disable eDMA */
-	dw_edma_core_off(dw);
+	if (!(chip->flags & DW_EDMA_CHIP_PARTIAL))
+		dw_edma_core_off(dw);
 
 	/* Free irqs */
 	for (i = (dw->nr_irqs - 1); i >= 0; i--)
diff --git a/include/linux/dma/edma.h b/include/linux/dma/edma.h
index 2bf2298711e1..84f0e728d300 100644
--- a/include/linux/dma/edma.h
+++ b/include/linux/dma/edma.h
@@ -55,9 +55,15 @@ enum dw_edma_map_format {
 /**
  * enum dw_edma_chip_flags - Flags specific to an eDMA chip
  * @DW_EDMA_CHIP_LOCAL:		eDMA is used locally by an endpoint
+ * @DW_EDMA_CHIP_PARTIAL:	Only channels described by this instance are
+ *				owned by this driver. Controller-wide state
+ *				must be preserved, and layouts with shared
+ *				direction-wide registers must only be shared at
+ *				direction granularity.
  */
 enum dw_edma_chip_flags {
 	DW_EDMA_CHIP_LOCAL	= BIT(0),
+	DW_EDMA_CHIP_PARTIAL	= BIT(1),
 };
 
 /**
-- 
2.51.0