[PATCH v2] dmaengine: mediatek: uart: synchronize completion callbacks

[James Hilliard <james.hilliard1@gmail.com>]

Terminating a UART APDMA channel waits for its interrupt handler, but
completion callbacks run in the virt-dma tasklet. A callback already
removed from desc_completed can still be running after termination
returns. With no device_synchronize hook, dmaengine_synchronize() does
not wait for that callback either.

Add vchan_dma_synchronize() with the DMAengine callback signature and
use it for UART APDMA. The shared helper wraps vchan_synchronize() so
drivers can assign it directly to device_synchronize. Document that the
termination path must have stopped the channel and prevented new
callbacks before using it.

This lets clients finish asynchronous cancellation before reusing
callback state or freeing DMA buffers. Keep the callback wait out of
device_terminate_all so it can be performed without locks acquired by
the callback.

Fixes: 9135408c3ace ("dmaengine: mediatek: Add MediaTek UART APDMA support")
Suggested-by: Frank Li <Frank.li@oss.nxp.com>
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
Changes in v2:
- Add a documented vchan_dma_synchronize(struct dma_chan *) helper in
  virt-dma.h and use it directly as the UART APDMA device_synchronize
  callback, following Frank Li's review [0].
- Link to v1: https://patch.msgid.link/20261021-submit-mtk-uart-dma-synchronize-v1-v1-1-4f7718cfef41@gmail.com

[0] https://lore.kernel.org/r/atoz6M7RHU14_a3J@SMW015318/
---
 drivers/dma/mediatek/mtk-uart-apdma.c |  1 +
 drivers/dma/virt-dma.h                | 15 +++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/drivers/dma/mediatek/mtk-uart-apdma.c b/drivers/dma/mediatek/mtk-uart-apdma.c
index f74e9a328588..6bc4f20d8b06 100644
--- a/drivers/dma/mediatek/mtk-uart-apdma.c
+++ b/drivers/dma/mediatek/mtk-uart-apdma.c
@@ -514,6 +514,7 @@ static int mtk_uart_apdma_probe(struct platform_device *pdev)
 	mtkd->ddev.device_config = mtk_uart_apdma_slave_config;
 	mtkd->ddev.device_pause = mtk_uart_apdma_device_pause;
 	mtkd->ddev.device_terminate_all = mtk_uart_apdma_terminate_all;
+	mtkd->ddev.device_synchronize = vchan_dma_synchronize;
 	mtkd->ddev.src_addr_widths = BIT(DMA_SLAVE_BUSWIDTH_1_BYTE);
 	mtkd->ddev.dst_addr_widths = BIT(DMA_SLAVE_BUSWIDTH_1_BYTE);
 	mtkd->ddev.directions = BIT(DMA_DEV_TO_MEM) | BIT(DMA_MEM_TO_DEV);
diff --git a/drivers/dma/virt-dma.h b/drivers/dma/virt-dma.h
index 59d9eabc8b67..f1fc3bf48275 100644
--- a/drivers/dma/virt-dma.h
+++ b/drivers/dma/virt-dma.h
@@ -234,4 +234,19 @@ static inline void vchan_synchronize(struct virt_dma_chan *vc)
 	vchan_dma_desc_free_list(vc, &head);
 }
 
+/**
+ * vchan_dma_synchronize - DMAengine callback to synchronize a virtual channel
+ * @chan: DMA channel to synchronize
+ *
+ * Use as device_synchronize when device_terminate_all has stopped the channel
+ * and prevented new callbacks from being scheduled. This waits for callbacks
+ * and frees terminated descriptors through vchan_synchronize().
+ *
+ * Context: non-atomic context, with no locks held that callbacks need.
+ */
+static inline void vchan_dma_synchronize(struct dma_chan *chan)
+{
+	vchan_synchronize(to_virt_chan(chan));
+}
+
 #endif

---
base-commit: 93f51579e7df248780214094418f205253383cc5
change-id: 20261021-submit-mtk-uart-dma-synchronize-v1-158c8303b574

Best regards,
--  
James Hilliard <james.hilliard1@gmail.com>