[PATCH v1 1/1] Revert "serial: 8250_port: Drop duplicate NULL check"

[PATCH v1 1/1] Revert "serial: 8250_port: Drop duplicate NULL check"

[Ferry Toth]

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@vger.kernel.org
Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>
---
 drivers/tty/serial/8250/8250_port.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 38fa45e74a37..19d0663ee756 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2428,8 +2428,8 @@ 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) {
-- 
2.53.0

Re: [PATCH v1 1/1] Revert "serial: 8250_port: Drop duplicate NULL check"

[Greg Kroah-Hartman]

On Sat, Oct 10, 2026 at 09:37:56PM +0200, Ferry Toth wrote:
> 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.

Ugh, sorry about missing that.  I'll queue this up when I get a chance
in the next few days, thanks for posting it.

greg k-h

Re: [PATCH v1 1/1] Revert "serial: 8250_port: Drop duplicate NULL check"

[Ferry Toth]

Hi

Op 11-10-2026 om 07:35 schreef Greg Kroah-Hartman:
> On Sat, Oct 10, 2026 at 09:37:56PM +0200, Ferry Toth wrote:
>> 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.
> Ugh, sorry about missing that.  I'll queue this up when I get a chance
> in the next few days, thanks for posting it.
>
> greg k-h

No worries, sorry I found it this late. I have been behind testing 
kernels and only now trying to catch up.

Ferry