From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sebastian Andrzej Siewior Subject: Re: [PATCH 1/3] tty: serial: 8250_omap: do not use RX DMA if pause is not supported Date: Mon, 10 Aug 2015 14:19:37 +0200 Message-ID: <55C896D9.5040300@linutronix.de> References: <1438977619-15488-1-git-send-email-bigeasy@linutronix.de> <1438977619-15488-2-git-send-email-bigeasy@linutronix.de> <55C890E2.50208@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <55C890E2.50208@ti.com> Sender: linux-kernel-owner@vger.kernel.org To: Peter Ujfalusi , Vinod Koul , Russell King , peter@hurleysoftware.com Cc: Dan Williams , dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org, nsekhar@ti.com, linux-omap@vger.kernel.org, linux-serial@vger.kernel.org, john.ogness@linutronix.de List-Id: linux-omap@vger.kernel.org On 08/10/2015 01:54 PM, Peter Ujfalusi wrote: >> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c >> index 0340ee6ba970..07a11e0935e4 100644 >> --- a/drivers/tty/serial/8250/8250_omap.c >> @@ -769,7 +771,9 @@ static void omap_8250_rx_dma_flush(struct uart_8250_port *p) >> return; >> } >> >> - dmaengine_pause(dma->rxchan); >> + ret = dmaengine_pause(dma->rxchan); >> + if (WARN_ON_ONCE(ret)) >> + priv->rx_dma_broken = true; > > I don't think this is good thing for the stable _and_ for the mainline at the > same time: > in stable the rx DMA should not be allowed since the stable kernels does not > allow pause/resume with omap-dma, so there the rx DMA should be just disabled > for UART. This change will cause regression since it introduce a WARN_ON_ONCE, > which will be printed if the user tries to use non working feature. Okay. We do have pause support in mainline for edma since v4.2-rc1. This driver can use edma or sdma depending on the configuration. But it is not yet released. So you suggest remove RX-DMA support completely from the 8250-omap, mark it stable, and revert that patch once we have it fixed in sdma? > In mainline you will eventually going to have pause/resume support so this > patch will make no sense there. The way this works is that it has to be fixed upstream before it can be backported stable. Also Russell made clear (for a good reason) that the RX problem has to be fixed upstream before he thinks about acking the SDMA patch. So if you prefer to instead remove RX-DMA until it is fixed, I am all yours. Sebastian