linux-kernel - Re: [PATCH v3 3/9] tty: serial: fsl_lpuart: handle EPROBE

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Date:   Tue, 3 Mar 2020 12:47:37 -0600
From:   Rob Herring <robh@...nel.org>
To:     Michael Walle <michael@...le.cc>
Cc:     linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Jiri Slaby <jslaby@...e.com>, Peng Fan <peng.fan@....com>,
        Vabhav Sharma <vabhav.sharma@....com>,
        Yuan Yao <yao.yuan@....com>
Subject: Re: [PATCH v3 3/9] tty: serial: fsl_lpuart: handle EPROBE_DEFER for
 DMA

On Tue, Mar 03, 2020 at 06:43:00PM +0100, 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.

The subject should be updated as this doesn't involve deferred probe any 
more. 

> 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.
> 
> To workaround this, we request the DMA channel in _startup(). Other
> serial drivers do it the same way.
> 
> Signed-off-by: Michael Walle <michael@...le.cc>
> ---
>  drivers/tty/serial/fsl_lpuart.c | 84 +++++++++++++++++++++------------
>  1 file changed, 53 insertions(+), 31 deletions(-)
> 
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index c31b8f3db6bf..0b8c477b32a3 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -1493,36 +1493,63 @@ 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_slave_channel(sport->port.dev, "tx");
> +	if (!sport->dma_tx_chan) {
> +		dev_info_once(sport->port.dev,
> +			      "DMA tx channel request failed, operating without tx DMA\n");

Might be useful to print the errno too.

Rob