From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Ferry Toth <ftoth@exalondelft.nl>
Cc: "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>,
"John Ogness" <john.ogness@linutronix.de>,
"Crescent Hsieh" <crescentcy.hsieh@moxa.com>,
"Hugo Villeneuve" <hvilleneuve@dimonoff.com>,
linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org,
"Jiri Slaby" <jirislaby@kernel.org>,
stable@vger.kernel.org
Subject: Re: [PATCH v1 1/1] Revert "serial: 8250_port: Drop duplicate NULL check"
Date: Mon, 12 Oct 2026 23:28:02 +0300 [thread overview]
Message-ID: <as1C0qoAwkBBCEcC@ashevche-desk.local> (raw)
In-Reply-To: <20261010193755.187706-2-ftoth@exalondelft.nl>
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.
As discussed off-list with Ferry the culprit patch was inspired
by 8250_omap implementation which may have the same bug lurking.
Thanks, Ferry, for a good catch!
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
--
With Best Regards,
Andy Shevchenko
prev parent reply other threads:[~2026-10-12 20:28 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-10 19:37 [PATCH v1 1/1] Revert "serial: 8250_port: Drop duplicate NULL check" Ferry Toth
2026-10-10 19:49 ` sashiko-bot
2026-10-11 5:35 ` Greg Kroah-Hartman
2026-10-11 19:28 ` Ferry Toth
2026-10-12 20:28 ` Andy Shevchenko [this message]
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