Re: [PATCH v1 1/1] Revert "serial: 8250_port: Drop duplicate NULL check"
From: Ferry Toth
Date: Sun Oct 11 2026 - 15:30:58 EST
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.Ugh, sorry about missing that. I'll queue this up when I get a chance
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.
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