Re: [PATCH v2] iio: adc: stm32-adc: enable timestamping for non-DMA usage

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Jonathan,

On 24.01.21 16:22, Jonathan Cameron wrote:
> On Fri, 22 Jan 2021 12:33:55 +0100
> Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
> 
>> For non-DMA usage, we have an easy way to associate a timestamp with a
>> sample: iio_pollfunc_store_time stores a timestamp in the primary
>> trigger IRQ handler and stm32_adc_trigger_handler runs in the IRQ thread
>> to push out the buffer along with the timestamp.
>>
>> For this to work, the driver needs to register an IIO_TIMESTAMP channel.
>> Do this.
>>
>> For DMA, it's not as easy, because we don't push the buffers out of
>> stm32_adc_trigger, but out of stm32_adc_dma_buffer_done, which runs in
>> a tasklet scheduled after a DMA completion.
>>
>> Preferably, the DMA controller would copy us the timestamp into that buffer
>> as well. Until this is implemented, restrict timestamping support to
>> only PIO. For low-frequency sampling, PIO is probably good enough.
>>
>> Cc: Holger Assmann <has@pengutronix.de>
>> Acked-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
>> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> 
> This patch itself is fine, but it will expose a potential bug.
> 
> The buffer passed to iio_push_to_buffers_with_timestamp needs to be suitably
> aligned to take an 8 byte timestamp and large enough to do so.
>  Currently, in this driver it isn't.
> 	u16			buffer[STM32_ADC_MAX_SQ];
> Appears to be the same length as the channel count, and isn't 8 byte
> aligned. (add __aligned(8) to fix that)
> 
> Could you add that fix to this patch as well?

Just done so, thanks. But I think it's very surprising API to expect a void *
to have a specific alignment. Should this perhaps be encoded into
the function signature? e.g.

typedef void __aligned_u64_void __aligned(8);
static inline int iio_push_to_buffers_with_timestamp(struct iio_dev *indio_dev,
        __aligned_u64_void *data, int64_t timestamp);

[I assume put_unaligned_* isn't used for performance reasons?)

Cheers,
Ahmad

> 
> Thanks,
> 
> Jonathan
> 
> 
>> ---
>> v1 -> v2:
>>   - Added comment about timestamping being PIO only (Fabrice)
>>   - Added missing DMA resource clean up in error path (Fabrice)
>>   - Added Fabrice's Acked-by
>> ---
>>  drivers/iio/adc/stm32-adc.c | 35 +++++++++++++++++++++++++++++------
>>  1 file changed, 29 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/iio/adc/stm32-adc.c b/drivers/iio/adc/stm32-adc.c
>> index c067c994dae2..885bb514503c 100644
>> --- a/drivers/iio/adc/stm32-adc.c
>> +++ b/drivers/iio/adc/stm32-adc.c
>> @@ -1718,7 +1718,7 @@ static void stm32_adc_chan_init_one(struct iio_dev *indio_dev,
>>  	}
>>  }
>>  
>> -static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
>> +static int stm32_adc_chan_of_init(struct iio_dev *indio_dev, bool timestamping)
>>  {
>>  	struct device_node *node = indio_dev->dev.of_node;
>>  	struct stm32_adc *adc = iio_priv(indio_dev);
>> @@ -1766,6 +1766,9 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
>>  		return -EINVAL;
>>  	}
>>  
>> +	if (timestamping)
>> +		num_channels++;
>> +
>>  	channels = devm_kcalloc(&indio_dev->dev, num_channels,
>>  				sizeof(struct iio_chan_spec), GFP_KERNEL);
>>  	if (!channels)
>> @@ -1816,6 +1819,19 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
>>  		stm32_adc_smpr_init(adc, channels[i].channel, smp);
>>  	}
>>  
>> +	if (timestamping) {
>> +		struct iio_chan_spec *timestamp = &channels[scan_index];
>> +
>> +		timestamp->type = IIO_TIMESTAMP;
>> +		timestamp->channel = -1;
>> +		timestamp->scan_index = scan_index;
>> +		timestamp->scan_type.sign = 's';
>> +		timestamp->scan_type.realbits = 64;
>> +		timestamp->scan_type.storagebits = 64;
>> +
>> +		scan_index++;
>> +	}
>> +
>>  	indio_dev->num_channels = scan_index;
>>  	indio_dev->channels = channels;
>>  
>> @@ -1875,6 +1891,7 @@ static int stm32_adc_probe(struct platform_device *pdev)
>>  	struct device *dev = &pdev->dev;
>>  	irqreturn_t (*handler)(int irq, void *p) = NULL;
>>  	struct stm32_adc *adc;
>> +	bool timestamping = false;
>>  	int ret;
>>  
>>  	if (!pdev->dev.of_node)
>> @@ -1931,16 +1948,22 @@ static int stm32_adc_probe(struct platform_device *pdev)
>>  	if (ret < 0)
>>  		return ret;
>>  
>> -	ret = stm32_adc_chan_of_init(indio_dev);
>> -	if (ret < 0)
>> -		return ret;
>> -
>>  	ret = stm32_adc_dma_request(dev, indio_dev);
>>  	if (ret < 0)
>>  		return ret;
>>  
>> -	if (!adc->dma_chan)
>> +	if (!adc->dma_chan) {
>> +		/* For PIO mode only, iio_pollfunc_store_time stores a timestamp
>> +		 * in the primary trigger IRQ handler and stm32_adc_trigger_handler
>> +		 * runs in the IRQ thread to push out buffer along with timestamp.
>> +		 */
>>  		handler = &stm32_adc_trigger_handler;
>> +		timestamping = true;
>> +	}
>> +
>> +	ret = stm32_adc_chan_of_init(indio_dev, timestamping);
>> +	if (ret < 0)
>> +		goto err_dma_disable;
>>  
>>  	ret = iio_triggered_buffer_setup(indio_dev,
>>  					 &iio_pollfunc_store_time, handler,
> 
> 

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