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

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Date:   Tue, 03 Mar 2020 21:57:42 +0100
From:   Michael Walle <michael@...le.cc>
To:     Rob Herring <robh@...nel.org>
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

Am 3. März 2020 19:47:37 MEZ schrieb Rob Herring <robh@...nel.org>:
>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. 

ok

>
>> 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.

I didn't want to change the original error message. But I can change that when I update the subject. 

-michael