[PATCH 4/4] serial: amba-pl011: Respect DMA controller's copy_align requirement

[Kartik Rajput <kkartik@nvidia.com>]

Some DMA controllers require transfer lengths to be aligned to a
specific boundary. For example, the Tegra GPC DMA requires 4-byte
(word) aligned transfers and will reject unaligned lengths.

Align the TX DMA buffer length down to the DMA controller's copy_align
boundary before submitting the transfer. Any remaining unaligned bytes
will be transmitted via PIO on subsequent calls, which is the existing
fallback behavior when DMA is not used.

Signed-off-by: Kartik Rajput <kkartik@nvidia.com>
---
 drivers/tty/serial/amba-pl011.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 7f8deb30650a..98e434b0c30a 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -649,6 +649,15 @@ static int pl011_dma_tx_refill(struct uart_amba_port *uap)
 		count = PL011_DMA_BUFFER_SIZE;
 
 	count = kfifo_out_peek(&tport->xmit_fifo, dmatx->buf, count);
+
+	/*
+	 * Align the TX buffer length to the DMA controller's copy_align
+	 * requirements. Some DMA controllers (e.g., Tegra GPC DMA) require
+	 * word-aligned transfers. Unaligned bytes will be sent via PIO.
+	 */
+	if (chan->device->copy_align)
+		count = ALIGN_DOWN(count, 1 << chan->device->copy_align);
+
 	dmatx->len = count;
 	dmatx->dma = dma_map_single(dma_dev->dev, dmatx->buf, count,
 				    DMA_TO_DEVICE);
-- 
2.43.0