serial: amba-pl011: Respect DMA controller's copy_align requirement

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>
Link: https://patch.msgid.link/20260225065915.341522-5-kkartik@nvidia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index b604274c17910d76612530e0022bceec568a88ca..028e37ad8d79f89296f1601a1deffe0730d15a9d 100644 (file)
--- 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);