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

[Ferry Toth <fntoth@gmail.com>]

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