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Revert "serial: 8250_port: Drop duplicate NULL check"

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This reverts commit 7885af04df.

Commit 7885af04df ("serial: 8250_port: Drop duplicate NULL check")
added `up->dma = NULL;` to serial8250_release_dma(), with the stated
intent of making sure DMA would not be used again after release.

However, up->dma is assigned once at probe time.  serial8250_do_startup()
only calls serial8250_request_dma() when up->dma is non-NULL, so after
the first close the pointer stays NULL and DMA is never re-acquired.

On Intel Tangier (Edison) with HSU DMA this causes a regression: the
first port open uses TX DMA, the second and later opens transmit in PIO
mode, producing 64-byte FIFO bursts with ~50us gaps between them.

Fixes: 7885af04df ("serial: 8250_port: Drop duplicate NULL check")
Cc: stable@kernel.org
Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20261010193755.187706-2-ftoth@exalondelft.nl
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Ferry Toth 2026-10-10 21:37:56 +02:00 committed by Greg Kroah-Hartman
parent d6a0791098
commit 6ae994d513
1 changed files with 2 additions and 2 deletions
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4
drivers/tty/serial/8250/8250_port.c
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@ -2452,8 +2452,8 @@ void serial8250_do_shutdown(struct uart_port *port)
synchronize_irq(port->irq);
serial8250_release_dma(up);
up->dma = NULL;
if (up->dma)
serial8250_release_dma(up);
scoped_guard(uart_port_lock_irqsave, port) {
if (port->flags & UPF_FOURPORT) {