[PATCH v2 4/5] dmaengine: dw-edma: HDMA: Add sync read before starting the DMA transfer in remote setup

["Köry Maincent" <kory.maincent@bootlin.com>]

From: Kory Maincent <kory.maincent@bootlin.com>

The Linked list element and pointer are not stored in the same memory as
the HDMA controller register. If the doorbell register is toggled before
the full write of the linked list a race condition error can appears.
In remote setup we can only use a readl to the memory to assured the full
write has occurred.

Fixes: e74c39573d35 ("dmaengine: dw-edma: Add support for native HDMA")
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
---

Changes in v2:
- Move the sync read in a function.
- Add commments
---
 drivers/dma/dw-edma/dw-hdma-v0-core.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/dma/dw-edma/dw-hdma-v0-core.c b/drivers/dma/dw-edma/dw-hdma-v0-core.c
index 0cce1880cfdc..26b5020dcc2a 100644
--- a/drivers/dma/dw-edma/dw-hdma-v0-core.c
+++ b/drivers/dma/dw-edma/dw-hdma-v0-core.c
@@ -221,6 +221,25 @@ static void dw_hdma_v0_core_write_chunk(struct dw_edma_chunk *chunk)
 	dw_hdma_v0_write_ll_link(chunk, i, control, chunk->ll_region.paddr);
 }
 
+/**
+ * dw_hdma_v0_sync_ll_data() - sync the ll data write
+ * @chunk: dma chunk
+ *
+ * In case of remote HDMA engine setup, the DW PCIe RP/EP internals
+ * configuration registers and Application memory are normally accesse
+ * over different buses. We need to insure ll data has been written before
+ * toggling the doorbell register.
+ */
+static void dw_hdma_v0_sync_ll_data(struct dw_edma_chunk *chunk)
+{
+	if (!(chunk->chan->dw->chip->flags & DW_EDMA_CHIP_LOCAL))
+		/* Linux memory barriers don't cater for what's required here.
+		 * What's required is what's here - a read of the linked
+		 * list region.
+		 */
+		readl(chunk->ll_region.vaddr.io);
+}
+
 static void dw_hdma_v0_core_start(struct dw_edma_chunk *chunk, bool first)
 {
 	struct dw_edma_chan *chan = chunk->chan;
@@ -251,6 +270,9 @@ static void dw_hdma_v0_core_start(struct dw_edma_chunk *chunk, bool first)
 	/* Set consumer cycle */
 	SET_CH_32(dw, chan->dir, chan->id, cycle_sync,
 		  HDMA_V0_CONSUMER_CYCLE_STAT | HDMA_V0_CONSUMER_CYCLE_BIT);
+
+	dw_hdma_v0_sync_ll_data(chunk);
+
 	/* Doorbell */
 	SET_CH_32(dw, chan->dir, chan->id, doorbell, HDMA_V0_DOORBELL_START);
 }
-- 
2.25.1