On Mon, 25 Jan 2021 12:21:35 +0100
Ahmad Fatoum [off-list ref] wrote:

Hello Jonathan,

On 24.01.21 16:22, Jonathan Cameron wrote:

quoted

On Fri, 22 Jan 2021 12:33:55 +0100
Ahmad Fatoum [off-list ref] wrote:
  

quoted

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 <redacted>
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);

Hmm. Didn't realise you could put __aligned() on a parameter.  May well make sense
but we can't do it yet.   There are a few left over drivers that don't yet
guarantee the alignment.  On some architectures that's fine.  Once we've
done a final pass and fixed those up, we can tidy this up.
Note that we had this problem for a good 5-10 years before one report of actually
running into a problem.

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

No, it's actually about expectations of where that buffer might go.  There
are lots of potential consumers and many of them can assume a fixed
structure and hence assume alignment.

I agree, it's a very odd ABI requirement :(
Would probably have done things differently if we'd registered the built in
alignment issue earlier.  

Jonathan

Cheers,
Ahmad

quoted

Thanks,

Jonathan

  

quoted

---
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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