linux-kernel - Re: [PATCH 03/13] serial: 8250_dma: Support for deferred probing when requesting DMA channels

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Date:	Wed, 27 May 2015 13:58:25 +0300
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Tony Lindgren <tony@...mide.com>
CC:	<vinod.koul@...el.com>, <devicetree@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <dan.j.williams@...el.com>,
	<dmaengine@...r.kernel.org>, <linux-serial@...r.kernel.org>,
	<linux-omap@...r.kernel.org>, <linux-mmc@...r.kernel.org>,
	<linux-crypto@...r.kernel.org>, <linux-spi@...r.kernel.org>,
	<linux-media@...r.kernel.org>, <alsa-devel@...a-project.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 03/13] serial: 8250_dma: Support for deferred probing
 when requesting DMA channels

On 05/26/2015 06:08 PM, Tony Lindgren wrote:
> * Peter Ujfalusi <peter.ujfalusi@...com> [150526 06:28]:
>> Switch to use ma_request_slave_channel_compat_reason() to request the DMA
>> channels. In case of error, return the error code we received including
>> -EPROBE_DEFER
>>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
>> CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> ---
>>  drivers/tty/serial/8250/8250_dma.c | 18 ++++++++----------
>>  1 file changed, 8 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
>> index 21d01a491405..a617eca4e97d 100644
>> --- a/drivers/tty/serial/8250/8250_dma.c
>> +++ b/drivers/tty/serial/8250/8250_dma.c
>> @@ -182,21 +182,19 @@ int serial8250_request_dma(struct uart_8250_port *p)
>>  	dma_cap_set(DMA_SLAVE, mask);
>>  
>>  	/* Get a channel for RX */
>> -	dma->rxchan = dma_request_slave_channel_compat(mask,
>> -						       dma->fn, dma->rx_param,
>> -						       p->port.dev, "rx");
>> -	if (!dma->rxchan)
>> -		return -ENODEV;
>> +	dma->rxchan = dma_request_slave_channel_compat_reason(mask, dma->fn,
>> +					dma->rx_param, p->port.dev, "rx");
>> +	if (IS_ERR(dma->rxchan))
>> +		return PTR_ERR(dma->rxchan);
>>  
>>  	dmaengine_slave_config(dma->rxchan, &dma->rxconf);
>>  
>>  	/* Get a channel for TX */
>> -	dma->txchan = dma_request_slave_channel_compat(mask,
>> -						       dma->fn, dma->tx_param,
>> -						       p->port.dev, "tx");
>> -	if (!dma->txchan) {
>> +	dma->txchan = dma_request_slave_channel_compat_reason(mask, dma->fn,
>> +					dma->tx_param, p->port.dev, "tx");
>> +	if (IS_ERR(dma->txchan)) {
>>  		dma_release_channel(dma->rxchan);
>> -		return -ENODEV;
>> +		return PTR_ERR(dma->txchan);
>>  	}
>>  
>>  	dmaengine_slave_config(dma->txchan, &dma->txconf);
> 
> In general the drivers need to work just fine also without DMA.
> 
> Does this handle the case properly where no DMA channel is configured
> for the driver in the dts file?

The 8250 core will fall back to PIO mode if the DMA can not be requested.
At the morning I was looking at the 8250 stack and realized that
serial8250_request_dma() will not be called at driver probe time so this patch
can be ignored and will be dropped from the v2 series.

-- 
Péter
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