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

This reverts commit 7885af04df6e4efd72910200c1bfc079d61202e4.

Commit 7885af04df6e ("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: 7885af04df6e ("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>

0 files changed, 0 insertions, 0 deletions