Re: [PATCH] iio: adc: stm32-adc: Use dma_request_chan() instead dma_request_slave_channel()

Fabrice Gasnier
2020-01-06T10:25:31Z

On 12/23/19 4:42 PM, Jonathan Cameron wrote:
> On Tue, 17 Dec 2019 09:51:53 +0200
> Peter Ujfalusi <peter.ujfalusi@ti.com> wrote:
>
>> dma_request_slave_channel() is a wrapper on top of dma_request_chan()
>> eating up the error code.
>>
>> By using dma_request_chan() directly the driver can support deferred
>> probing against DMA.
>>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
> Hi Peter,
>
> Change looks sensible to me, but I'd like to leave a little longer
> for others to look at this.
>
> Give me a poke if I seem to have lost it by the end of the first
> week in Jan.
>
> Thanks,
>
> Jonathan
>
>> ---
>> drivers/iio/adc/stm32-adc.c | 10 ++++++++--
>> 1 file changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/iio/adc/stm32-adc.c b/drivers/iio/adc/stm32-adc.c
>> index 3b291d72701c..5dab23f1fdee 100644
>> --- a/drivers/iio/adc/stm32-adc.c
>> +++ b/drivers/iio/adc/stm32-adc.c
>> @@ -1746,9 +1746,15 @@ static int stm32_adc_dma_request(struct iio_dev *indio_dev)
>> struct dma_slave_config config;
>> int ret;
>>
>> - adc->dma_chan = dma_request_slave_channel(&indio_dev->dev, "rx");
>> - if (!adc->dma_chan)
>> + adc->dma_chan = dma_request_chan(&indio_dev->dev, "rx");
>> + if (IS_ERR(adc->dma_chan)) {
>> + if (PTR_ERR(adc->dma_chan) == -EPROBE_DEFER)
>> + return -EPROBE_DEFER;

Hi Peter,

Thanks for the patch and sorry for the late reply.

I'd rather prefer to check explicitly for -ENODEV (as the DMA is
optional) to use the IRQ mode and treat all other codes (including
EPROBE_DEFER) as errors.

Rationale is: This can hide other errors e.g. like all DMA channels are
busy/reserved for other usage. So the user may wrongly think it's
probing the driver, with DMA. This may be a corner case, but still...
DMA channels are assigned via device tree. I'd rather prefer to avoid
depending of runtime (probe ordering) in such a case.

Can you update the patch considering this ?

Please find here an alternate proposal:

adc->dma_chan = dma_request_chan(&indio_dev->dev, "rx");
if (IS_ERR(adc->dma_chan)) {
if (PTR_ERR(adc->dma_chan) != -ENODEV)
return PTR_ERR(adc->dma_chan);

/* DMA is optional: fall back to IRQ mode */
adc->dma_chan = NULL;
return 0;
}

Best regards,
Fabrice

>> +
>> + /* Ignore errors to fall back to IRQ mode */
>> + adc->dma_chan = NULL;
>> return 0;
>> + }
>>
>> adc->rx_buf = dma_alloc_coherent(adc->dma_chan->device->dev,
>> STM32_DMA_BUFFER_SIZE,
>