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Date:   Tue, 24 Mar 2020 17:17:40 +0100
From:   Michael Walle <michael@walle.cc>
To:     Andy Duan <fugang.duan@nxp.com>
Cc:     Leonard Crestez <leonard.crestez@nxp.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shawn Guo <shawnguo@kernel.org>, linux-serial@vger.kernel.org,
        linux-kernel@vger.kernel.org, Jiri Slaby <jslaby@suse.com>,
        Rob Herring <robh+dt@kernel.org>,
        Aisheng Dong <aisheng.dong@nxp.com>,
        Vabhav Sharma <vabhav.sharma@nxp.com>,
        Andrey Smirnov <andrew.smirnov@gmail.com>,
        Peng Fan <peng.fan@nxp.com>
Subject: Re: [PATCH v4 1/4] tty: serial: fsl_lpuart: fix DMA operation when
 using IOMMU
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References: <20200306214433.23215-1-michael@walle.cc>
 <20200306214433.23215-2-michael@walle.cc>
 <VI1PR04MB69413E158203E33D42E3B3B3EEF10@VI1PR04MB6941.eurprd04.prod.outlook.com>
 <VI1PR0402MB36001AEE78E0F067D29F2816FFF10@VI1PR0402MB3600.eurprd04.prod.outlook.com>
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Am 2020-03-24 17:12, schrieb Andy Duan:
> From: Leonard Crestez <leonard.crestez@nxp.com> Sent: Tuesday, March
> 24, 2020 11:27 PM
>> On 06.03.2020 23:44, Michael Walle wrote:
>> > The DMA channel might not be available at probe time. This is esp. the
>> > case if the DMA controller has an IOMMU mapping.
>> >
>> > There is also another caveat. If there is no DMA controller at all,
>> > dma_request_chan() will also return -EPROBE_DEFER. Thus we cannot test
>> > for -EPROBE_DEFER in probe(). Otherwise the lpuart driver will fail to
>> > probe if, for example, the DMA driver is not enabled in the kernel
>> > configuration.
> If DMA driver is not enabled, we should disable DMA controller node in
> dts file to match current sw environment, then driver doesn't do defer 
> probe.
> 
> So I still suggest to check -EPROBE_DEFER for 
> dma_request_slave_channel() in
> .probe() function.

I don't know if I can follow you here. This would lead to non functional 
setups,
eg. one build its own kernel with DMA disabled, but still have a device 
tree
with the DMA nodes. And besides, the current workaround to request the 
DMA
channel in startup() is basically working, isn't it? And once the 
underlying
problem is fixed (the infinite EPROBE_DEFER), it could be moved back 
into
_probe().

-michael

> 
> Andy
>> >
>> > To workaround this, we request the DMA channel in _startup(). Other
>> > serial drivers do it the same way.
>> >
>> > Signed-off-by: Michael Walle <michael@walle.cc>
>> 
>> This appears to cause boot hangs on imx8qxp-mek (boards boots fine on
>> next-20200324 if this patch is reverted)
>> 
>> > ---
>> >   drivers/tty/serial/fsl_lpuart.c | 88 +++++++++++++++++++++------------
>> >   1 file changed, 57 insertions(+), 31 deletions(-)
>> >
>> > diff --git a/drivers/tty/serial/fsl_lpuart.c
>> > b/drivers/tty/serial/fsl_lpuart.c index c31b8f3db6bf..33798df4d727
>> > 100644
>> > --- a/drivers/tty/serial/fsl_lpuart.c
>> > +++ b/drivers/tty/serial/fsl_lpuart.c
>> > @@ -1493,36 +1493,67 @@ static void rx_dma_timer_init(struct
>> lpuart_port *sport)
>> >   static void lpuart_tx_dma_startup(struct lpuart_port *sport)
>> >   {
>> >   	u32 uartbaud;
>> > +	int ret;
>> >
>> > -	if (sport->dma_tx_chan && !lpuart_dma_tx_request(&sport->port)) {
>> > -		init_waitqueue_head(&sport->dma_wait);
>> > -		sport->lpuart_dma_tx_use = true;
>> > -		if (lpuart_is_32(sport)) {
>> > -			uartbaud = lpuart32_read(&sport->port, UARTBAUD);
>> > -			lpuart32_write(&sport->port,
>> > -				       uartbaud | UARTBAUD_TDMAE, UARTBAUD);
>> > -		} else {
>> > -			writeb(readb(sport->port.membase + UARTCR5) |
>> > -				UARTCR5_TDMAS, sport->port.membase + UARTCR5);
>> > -		}
>> > +	sport->dma_tx_chan = dma_request_chan(sport->port.dev, "tx");
>> > +	if (IS_ERR(sport->dma_tx_chan)) {
>> > +		dev_info_once(sport->port.dev,
>> > +			      "DMA tx channel request failed, operating without tx
>> DMA (%ld)\n",
>> > +			      PTR_ERR(sport->dma_tx_chan));
>> 
>> It seems that this since this is called from lpuart32_startup with
>> &sport->port.lock held and lpuart32_console_write takes the same lock 
>> it can
>> and hang.
>> 
>> As a workaround I can just remove this print but there are other 
>> possible error
>> conditions in dmaengine code which can cause a printk.
>> 
>> Maybe the port lock should only be held around register manipulation?
>> 
>> > +		sport->dma_tx_chan = NULL;
>> > +		goto err;
>> > +	}
>> > +
>> > +	ret = lpuart_dma_tx_request(&sport->port);
>> > +	if (!ret)
>> > +		goto err;
>> 
>> This is backwards: lpuart_dma_tx_request returns negative errno on 
>> failure.
>> 
>> > +
>> > +	init_waitqueue_head(&sport->dma_wait);
>> > +	sport->lpuart_dma_tx_use = true;
>> > +	if (lpuart_is_32(sport)) {
>> > +		uartbaud = lpuart32_read(&sport->port, UARTBAUD);
>> > +		lpuart32_write(&sport->port,
>> > +			       uartbaud | UARTBAUD_TDMAE, UARTBAUD);
>> >   	} else {
>> > -		sport->lpuart_dma_tx_use = false;
>> > +		writeb(readb(sport->port.membase + UARTCR5) |
>> > +		       UARTCR5_TDMAS, sport->port.membase + UARTCR5);
>> >   	}
>> > +
>> > +	return;
>> > +
>> > +err:
>> > +	sport->lpuart_dma_tx_use = false;
>> >   }
>> >
>> >   static void lpuart_rx_dma_startup(struct lpuart_port *sport)
>> >   {
>> > -	if (sport->dma_rx_chan && !lpuart_start_rx_dma(sport)) {
>> > -		/* set Rx DMA timeout */
>> > -		sport->dma_rx_timeout = msecs_to_jiffies(DMA_RX_TIMEOUT);
>> > -		if (!sport->dma_rx_timeout)
>> > -			sport->dma_rx_timeout = 1;
>> > +	int ret;
>> >
>> > -		sport->lpuart_dma_rx_use = true;
>> > -		rx_dma_timer_init(sport);
>> > -	} else {
>> > -		sport->lpuart_dma_rx_use = false;
>> > +	sport->dma_rx_chan = dma_request_chan(sport->port.dev, "rx");
>> > +	if (IS_ERR(sport->dma_rx_chan)) {
>> > +		dev_info_once(sport->port.dev,
>> > +			      "DMA rx channel request failed, operating without rx
>> DMA (%ld)\n",
>> > +			      PTR_ERR(sport->dma_rx_chan));
>> > +		sport->dma_rx_chan = NULL;
>> > +		goto err;
>> >   	}
>> > +
>> > +	ret = lpuart_start_rx_dma(sport);
>> > +	if (ret)
>> > +		goto err;
>> 
>> This is not backwards.
>> 
>> > +
>> > +	/* set Rx DMA timeout */
>> > +	sport->dma_rx_timeout = msecs_to_jiffies(DMA_RX_TIMEOUT);
>> > +	if (!sport->dma_rx_timeout)
>> > +		sport->dma_rx_timeout = 1;
>> > +
>> > +	sport->lpuart_dma_rx_use = true;
>> > +	rx_dma_timer_init(sport);
>> > +
>> > +	return;
>> > +
>> > +err:
>> > +	sport->lpuart_dma_rx_use = false;
>> >   }
>> >
>> >   static int lpuart_startup(struct uart_port *port) @@ -1615,6
>> > +1646,11 @@ static void lpuart_dma_shutdown(struct lpuart_port *sport)
>> >   			dmaengine_terminate_all(sport->dma_tx_chan);
>> >   		}
>> >   	}
>> > +
>> > +	if (sport->dma_tx_chan)
>> > +		dma_release_channel(sport->dma_tx_chan);
>> > +	if (sport->dma_rx_chan)
>> > +		dma_release_channel(sport->dma_rx_chan);
>> >   }
>> >
>> >   static void lpuart_shutdown(struct uart_port *port) @@ -2520,16
>> > +2556,6 @@ static int lpuart_probe(struct platform_device *pdev)
>> >
>> >   	sport->port.rs485_config(&sport->port, &sport->port.rs485);
>> >
>> > -	sport->dma_tx_chan = dma_request_slave_channel(sport->port.dev,
>> "tx");
>> > -	if (!sport->dma_tx_chan)
>> > -		dev_info(sport->port.dev, "DMA tx channel request failed, "
>> > -				"operating without tx DMA\n");
>> > -
>> > -	sport->dma_rx_chan = dma_request_slave_channel(sport->port.dev,
>> "rx");
>> > -	if (!sport->dma_rx_chan)
>> > -		dev_info(sport->port.dev, "DMA rx channel request failed, "
>> > -				"operating without rx DMA\n");
>> > -
>> >   	return 0;
>> >
>> >   failed_attach_port:
>> >