[PATCH] dmaengine: mediatek: uart: make channel termination atomic

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

The 8250 transmit flush path calls dmaengine_terminate_async() with the
UART port spinlock held and interrupts disabled. UART APDMA termination
uses sleeping register polls and synchronize_irq(), which can also wait for
a threaded handler or for an interrupt pending on the calling CPU.

Serialize the stop sequence with the interrupt handler using the virtual
channel lock and use atomic register polling. A handler that was waiting
for that lock must check interrupt status before accessing a descriptor:
termination may already have masked and acknowledged its interrupt. This
allows termination to retire descriptors without waiting for the handler.

If the channel does not stop, return the timeout and retain its descriptors
instead of freeing state still owned by the transfer. Continue attempting
STOP after a flush timeout, since stopping the channel is sufficient for
termination even when the flush could not finish.

Fixes: 9135408c3ace ("dmaengine: mediatek: Add MediaTek UART APDMA support")
Cc: stable@vger.kernel.org
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 drivers/dma/mediatek/mtk-uart-apdma.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/dma/mediatek/mtk-uart-apdma.c b/drivers/dma/mediatek/mtk-uart-apdma.c
index f74e9a328588..73d4b701ac85 100644
--- a/drivers/dma/mediatek/mtk-uart-apdma.c
+++ b/drivers/dma/mediatek/mtk-uart-apdma.c
@@ -256,6 +256,12 @@ static irqreturn_t mtk_uart_apdma_irq_handler(int irq, void *dev_id)
 	unsigned long flags;
 
 	spin_lock_irqsave(&c->vc.lock, flags);
+	/* Termination may have acknowledged an interrupt before we got the lock. */
+	if (!mtk_uart_apdma_read(c, VFF_INT_FLAG)) {
+		spin_unlock_irqrestore(&c->vc.lock, flags);
+		return IRQ_NONE;
+	}
+
 	if (c->dir == DMA_DEV_TO_MEM)
 		mtk_uart_apdma_rx_handler(c);
 	else if (c->dir == DMA_MEM_TO_DEV)
@@ -400,10 +406,12 @@ static int mtk_uart_apdma_terminate_all(struct dma_chan *chan)
 	LIST_HEAD(head);
 	int ret;
 
+	spin_lock_irqsave(&c->vc.lock, flags);
+
 	mtk_uart_apdma_write(c, VFF_FLUSH, VFF_FLUSH_B);
 
-	ret = readx_poll_timeout(readl, c->base + VFF_FLUSH,
-			  status, status != VFF_FLUSH_B, 10, 100);
+	ret = readx_poll_timeout_atomic(readl, c->base + VFF_FLUSH,
+					status, status != VFF_FLUSH_B, 10, 100);
 	if (ret)
 		dev_err(c->vc.chan.device->dev, "flush: fail, status=0x%x\n",
 			mtk_uart_apdma_read(c, VFF_DEBUG_STATUS));
@@ -415,11 +423,14 @@ static int mtk_uart_apdma_terminate_all(struct dma_chan *chan)
 	 * 3. set stop as 0
 	 */
 	mtk_uart_apdma_write(c, VFF_STOP, VFF_STOP_B);
-	ret = readx_poll_timeout(readl, c->base + VFF_EN,
-			  status, !status, 10, 100);
-	if (ret)
+	ret = readx_poll_timeout_atomic(readl, c->base + VFF_EN,
+					status, !status, 10, 100);
+	if (ret) {
 		dev_err(c->vc.chan.device->dev, "stop: fail, status=0x%x\n",
 			mtk_uart_apdma_read(c, VFF_DEBUG_STATUS));
+		spin_unlock_irqrestore(&c->vc.lock, flags);
+		return ret;
+	}
 
 	mtk_uart_apdma_write(c, VFF_STOP, VFF_STOP_CLR_B);
 	mtk_uart_apdma_write(c, VFF_INT_EN, VFF_INT_EN_CLR_B);
@@ -429,9 +440,6 @@ static int mtk_uart_apdma_terminate_all(struct dma_chan *chan)
 	else if (c->dir == DMA_MEM_TO_DEV)
 		mtk_uart_apdma_write(c, VFF_INT_FLAG, VFF_TX_INT_CLR_B);
 
-	synchronize_irq(c->irq);
-
-	spin_lock_irqsave(&c->vc.lock, flags);
 	vchan_get_all_descriptors(&c->vc, &head);
 	spin_unlock_irqrestore(&c->vc.lock, flags);
 

---
base-commit: 93f51579e7df248780214094418f205253383cc5
change-id: 20261022-submit-mtk-uart-dma-terminate-atomic-v1-19693366ae4c

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