Re: [PATCH v2 3/5] iio: adc: at91-sama5d2_adc: add support for DMA

[Jonathan Cameron]

On Wed, 11 Oct 2017 09:35:30 +0300
Eugen Hristev <eugen.hristev@xxxxxxxxxxxxx> wrote:

> Added support for DMA transfers. The implementation uses the user watermark
> to decide whether DMA will be used or not. For watermark 1, DMA will not be
> used. If watermark is bigger, DMA will be used.
> Sysfs attributes are created to indicate whether the DMA is used,
> with hwfifo_enabled, and the current DMA watermark is readable
> in hwfifo_watermark. Minimum and maximum values are in hwfifo_watermark_min
> and hwfifo_watermark_max.
> 
> Signed-off-by: Eugen Hristev <eugen.hristev@xxxxxxxxxxxxx>

You have shrunk the size of the buffer passed to
iio_push_to_buffers_with_timestamp in the non DMA case such that it is
no longer big enough to take the timestamp (the data is used in place
in that call so we need the extra space).

Other than that (and what I think is a missleading dev_info) I think
this is looking pretty good.

Lars if you have time to look at this one that would be great as 
your knowledge of the dmaengine stuff is better than mine.

Jonathan

> ---
>   Changes in v2:
>  - No longer add last timestamp to all samples. Now, compute an interval
> between samples w.r.t. start and end time of the transfer and number
> of samples. Then distribute them each in the time interval.
>  - Add warning for conversion overrun. This helps user identify cases
> when the watermark needs adjustment : the software is too slow in reading
> data from the ADC.
>  - Protection around watermark is not needed, changing of the watermark
> cannot be done while the buffer is enabled. When buffer is disabled, all
> DMA resources are freed anyway.
>  - Added validation on trigger to be used by own device
>  - Best sample rate I could obtain using the low frequency clock was about
> 4k samples/second, with a watermark of 100. To get up to 50k samples/second
> the ADC frequency must be increased to max.
>  - Fixed computation of DMA buffer size
>  - Addressed other comments from mailing list review. Feedback is appreciated
> 
>  drivers/iio/adc/Kconfig            |   1 +
>  drivers/iio/adc/at91-sama5d2_adc.c | 432 +++++++++++++++++++++++++++++++++++--
>  2 files changed, 415 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/iio/adc/Kconfig b/drivers/iio/adc/Kconfig
> index 5762565..9ef288f 100644
> --- a/drivers/iio/adc/Kconfig
> +++ b/drivers/iio/adc/Kconfig
> @@ -158,6 +158,7 @@ config AT91_SAMA5D2_ADC
>  	tristate "Atmel AT91 SAMA5D2 ADC"
>  	depends on ARCH_AT91 || COMPILE_TEST
>  	depends on HAS_IOMEM
> +	depends on HAS_DMA
>  	select IIO_TRIGGERED_BUFFER
>  	help
>  	  Say yes here to build support for Atmel SAMA5D2 ADC which is
> diff --git a/drivers/iio/adc/at91-sama5d2_adc.c b/drivers/iio/adc/at91-sama5d2_adc.c
> index bc5b38e..7a9305a 100644
> --- a/drivers/iio/adc/at91-sama5d2_adc.c
> +++ b/drivers/iio/adc/at91-sama5d2_adc.c
> @@ -16,6 +16,8 @@
>  
>  #include <linux/bitops.h>
>  #include <linux/clk.h>
> +#include <linux/dma-mapping.h>
> +#include <linux/dmaengine.h>
>  #include <linux/interrupt.h>
>  #include <linux/io.h>
>  #include <linux/module.h>
> @@ -100,6 +102,8 @@
>  #define AT91_SAMA5D2_LCDR	0x20
>  /* Interrupt Enable Register */
>  #define AT91_SAMA5D2_IER	0x24
> +/* Interrupt Enable Register - general overrun error */
> +#define AT91_SAMA5D2_IER_GOVRE BIT(25)
>  /* Interrupt Disable Register */
>  #define AT91_SAMA5D2_IDR	0x28
>  /* Interrupt Mask Register */
> @@ -167,13 +171,16 @@
>  
>  /*
>   * Maximum number of bytes to hold conversion from all channels
> - * plus the timestamp
> + * without the timestamp.
>   */
>  #define AT91_BUFFER_MAX_BYTES ((AT91_SAMA5D2_SINGLE_CHAN_CNT +		\
> -				AT91_SAMA5D2_DIFF_CHAN_CNT) * 2 + 8)
> +				AT91_SAMA5D2_DIFF_CHAN_CNT) * 2)
>  
>  #define AT91_BUFFER_MAX_HWORDS (AT91_BUFFER_MAX_BYTES / 2)
>  
> +#define AT91_HWFIFO_MAX_SIZE_STR	"128"
> +#define AT91_HWFIFO_MAX_SIZE		128
> +
>  #define AT91_SAMA5D2_CHAN_SINGLE(num, addr)				\
>  	{								\
>  		.type = IIO_VOLTAGE,					\
> @@ -227,6 +234,28 @@ struct at91_adc_trigger {
>  	unsigned int			edge_type;
>  };
>  
> +/**
> + * at91_adc_dma - at91-sama5d2 dma information struct
> + * @dma_chan:		the dma channel acquired
> + * @rx_buf:		dma coherent allocated area
> + * @rx_dma_buf:		dma handler for the buffer
> + * @phys_addr:		physical address of the ADC base register
> + * @buf_idx:		index inside the dma buffer where reading was last done
> + * @rx_buf_sz:		size of buffer used by DMA operation
> + * @watermark:		number of conversions to copy before DMA triggers irq
> + * @dma_ts:		hold the start timestamp of dma operation
> + */
> +struct at91_adc_dma {
> +	struct dma_chan			*dma_chan;
> +	u8				*rx_buf;
> +	dma_addr_t			rx_dma_buf;
> +	phys_addr_t			phys_addr;
> +	int				buf_idx;
> +	int				rx_buf_sz;
> +	int				watermark;
> +	s64				dma_ts;
> +};
> +
>  struct at91_adc_state {
>  	void __iomem			*base;
>  	int				irq;
> @@ -241,6 +270,7 @@ struct at91_adc_state {
>  	u32				conversion_value;
>  	struct at91_adc_soc_info	soc_info;
>  	wait_queue_head_t		wq_data_available;
> +	struct at91_adc_dma		dma_st;
>  	u16				buffer[AT91_BUFFER_MAX_HWORDS];

I think this is no longer big enough...  For the normal triggered case
you still need enough room for the timestamp to be inserted in
iio_push_to_buffers_with_timestamp.

>  	/*
>  	 * lock to prevent concurrent 'single conversion' requests through
> @@ -312,11 +342,17 @@ static int at91_adc_configure_trigger(struct iio_trigger *trig, bool state)
>  		if (state) {
>  			at91_adc_writel(st, AT91_SAMA5D2_CHER,
>  					BIT(chan->channel));
> -			at91_adc_writel(st, AT91_SAMA5D2_IER,
> -					BIT(chan->channel));
> +			/* enable irq only if not using DMA */
> +			if (!st->dma_st.dma_chan) {
> +				at91_adc_writel(st, AT91_SAMA5D2_IER,
> +						BIT(chan->channel));
> +			}
>  		} else {
> -			at91_adc_writel(st, AT91_SAMA5D2_IDR,
> -					BIT(chan->channel));
> +			/* disable irq only if not using DMA */
> +			if (!st->dma_st.dma_chan) {
> +				at91_adc_writel(st, AT91_SAMA5D2_IDR,
> +						BIT(chan->channel));
> +			}
>  			at91_adc_writel(st, AT91_SAMA5D2_CHDR,
>  					BIT(chan->channel));
>  		}
> @@ -330,6 +366,10 @@ static int at91_adc_reenable_trigger(struct iio_trigger *trig)
>  	struct iio_dev *indio = iio_trigger_get_drvdata(trig);
>  	struct at91_adc_state *st = iio_priv(indio);
>  
> +	/* if we are using DMA, we must not reenable irq after each trigger */
> +	if (st->dma_st.dma_chan)
> +		return 0;
> +
>  	enable_irq(st->irq);
>  
>  	/* Needed to ACK the DRDY interruption */
> @@ -341,6 +381,153 @@ static const struct iio_trigger_ops at91_adc_trigger_ops = {
>  	.owner = THIS_MODULE,
>  	.set_trigger_state = &at91_adc_configure_trigger,
>  	.try_reenable = &at91_adc_reenable_trigger,
> +	.validate_device = iio_trigger_validate_own_device,
> +};
> +
> +static int at91_adc_dma_size_done(struct at91_adc_state *st)
> +{
> +	struct dma_tx_state state;
> +	enum dma_status status;
> +	int i, size;
> +
> +	status = dmaengine_tx_status(st->dma_st.dma_chan,
> +				     st->dma_st.dma_chan->cookie,
> +				     &state);
> +	if (status != DMA_IN_PROGRESS)
> +		return 0;
> +
> +	/* Transferred length is size in bytes from end of buffer */
> +	i = st->dma_st.rx_buf_sz - state.residue;
> +
> +	/* Return available bytes */
> +	if (i >= st->dma_st.buf_idx)
> +		size = i - st->dma_st.buf_idx;
> +	else
> +		size = st->dma_st.rx_buf_sz + i - st->dma_st.buf_idx;
> +	return size;
> +}
> +
> +static void at91_dma_buffer_done(void *data)
> +{
> +	struct iio_dev *indio_dev = data;
> +
> +	iio_trigger_poll_chained(indio_dev->trig);
> +}
> +
> +static int at91_adc_dma_start(struct iio_dev *indio_dev)
> +{
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +	struct dma_async_tx_descriptor *desc;
> +	dma_cookie_t cookie;
> +	int ret;
> +	u8 bit;
> +
> +	if (!st->dma_st.dma_chan)
> +		return 0;
> +
> +	/* we start a new DMA, so set buffer index to start */
> +	st->dma_st.buf_idx = 0;
> +
> +	/*
> +	 * compute buffer size w.r.t. watermark and enabled channels.
> +	 * scan_bytes is aligned so we need an exact size for DMA
> +	 */
> +	st->dma_st.rx_buf_sz = 0;
> +
> +	for_each_set_bit(bit, indio_dev->active_scan_mask,
> +			 indio_dev->num_channels) {
> +		struct iio_chan_spec const *chan = indio_dev->channels + bit;
> +
> +		st->dma_st.rx_buf_sz += chan->scan_type.storagebits / 8;
> +	}
> +	st->dma_st.rx_buf_sz *= st->dma_st.watermark;
> +
> +	/* Prepare a DMA cyclic transaction */
> +	desc = dmaengine_prep_dma_cyclic(st->dma_st.dma_chan,
> +					 st->dma_st.rx_dma_buf,
> +					 st->dma_st.rx_buf_sz,
> +					 st->dma_st.rx_buf_sz / 2,
> +					 DMA_DEV_TO_MEM, DMA_PREP_INTERRUPT);
> +
> +	if (!desc) {
> +		dev_err(&indio_dev->dev, "cannot prepare DMA cyclic\n");
> +		return -EBUSY;
> +	}
> +
> +	desc->callback = at91_dma_buffer_done;
> +	desc->callback_param = indio_dev;
> +
> +	cookie = dmaengine_submit(desc);
> +	ret = dma_submit_error(cookie);
> +	if (ret) {
> +		dev_err(&indio_dev->dev, "cannot submit DMA cyclic\n");
> +		dmaengine_terminate_async(st->dma_st.dma_chan);
> +		return ret;
> +	}
> +
> +	/* enable general overrun error signaling */
> +	at91_adc_writel(st, AT91_SAMA5D2_IER, AT91_SAMA5D2_IER_GOVRE);
> +	/* Issue pending DMA requests */
> +	dma_async_issue_pending(st->dma_st.dma_chan);
> +
> +	/* consider current time as DMA start time for timestamps */
> +	st->dma_st.dma_ts = iio_get_time_ns(indio_dev);
> +
> +	dev_dbg(&indio_dev->dev, "DMA cyclic started\n");
> +
> +	return 0;
> +}
> +
> +static int at91_adc_buffer_postenable(struct iio_dev *indio_dev)
> +{
> +	int ret;
> +
> +	ret = at91_adc_dma_start(indio_dev);
> +	if (ret) {
> +		dev_err(&indio_dev->dev, "buffer postenable failed\n");
> +		return ret;
> +	}
> +
> +	return iio_triggered_buffer_postenable(indio_dev);
> +}
> +
> +static int at91_adc_buffer_predisable(struct iio_dev *indio_dev)
> +{
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +	int ret;
> +	u8 bit;
> +
> +	ret = iio_triggered_buffer_predisable(indio_dev);
> +	if (ret < 0)
> +		dev_err(&indio_dev->dev, "buffer predisable failed\n");
> +
> +	if (!st->dma_st.dma_chan)
> +		return ret;
> +
> +	/* if we are using DMA we must clear registers and end DMA */
> +	dmaengine_terminate_sync(st->dma_st.dma_chan);
> +
> +	/*
> +	 * For each enabled channel we must read the last converted value
> +	 * to clear EOC status and not get a possible interrupt later.
> +	 * This value is being read by DMA from LCDR anyway
> +	 */
> +	for_each_set_bit(bit, indio_dev->active_scan_mask,
> +			 indio_dev->num_channels) {
> +		struct iio_chan_spec const *chan = indio_dev->channels + bit;
> +
> +		if (st->dma_st.dma_chan)
> +			at91_adc_readl(st, chan->address);
> +	}
> +
> +	/* read overflow register to clear possible overflow status */
> +	at91_adc_readl(st, AT91_SAMA5D2_OVER);
> +	return ret;
> +}
> +
> +static const struct iio_buffer_setup_ops at91_buffer_setup_ops = {
> +	.postenable = &at91_adc_buffer_postenable,
> +	.predisable = &at91_adc_buffer_predisable,
>  };
>  
>  static struct iio_trigger *at91_adc_allocate_trigger(struct iio_dev *indio,
> @@ -379,24 +566,71 @@ static int at91_adc_trigger_init(struct iio_dev *indio)
>  	return 0;
>  }
>  
> -static irqreturn_t at91_adc_trigger_handler(int irq, void *p)
> +static void at91_adc_trigger_handler_nodma(struct iio_dev *indio_dev,
> +					   struct iio_poll_func *pf)
>  {
> -	struct iio_poll_func *pf = p;
> -	struct iio_dev *indio = pf->indio_dev;
> -	struct at91_adc_state *st = iio_priv(indio);
> +	struct at91_adc_state *st = iio_priv(indio_dev);
>  	int i = 0;
>  	u8 bit;
>  
> -	for_each_set_bit(bit, indio->active_scan_mask, indio->num_channels) {
> -		struct iio_chan_spec const *chan = indio->channels + bit;
> +	for_each_set_bit(bit, indio_dev->active_scan_mask,
> +			 indio_dev->num_channels) {
> +		struct iio_chan_spec const *chan = indio_dev->channels + bit;
>  
>  		st->buffer[i] = at91_adc_readl(st, chan->address);
>  		i++;
>  	}
> +	iio_push_to_buffers_with_timestamp(indio_dev, st->buffer,
> +					   pf->timestamp);
> +}
> +
> +static void at91_adc_trigger_handler_dma(struct iio_dev *indio_dev)
> +{
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +	int transferred_len = at91_adc_dma_size_done(st);
> +	s64 ns = iio_get_time_ns(indio_dev);
> +	s64 interval;
> +	int sample_index = 0, sample_count, sample_size;
>  
> -	iio_push_to_buffers_with_timestamp(indio, st->buffer, pf->timestamp);
> +	sample_size = div_s64(st->dma_st.rx_buf_sz, st->dma_st.watermark);
>  
> -	iio_trigger_notify_done(indio->trig);
> +	sample_count = div_s64(transferred_len, sample_size);
> +
> +	/*
> +	 * interval between samples is total time since last transfer handling
> +	 * divided by the number of samples (total size divided by sample size)
> +	 */
> +	interval = div_s64((ns - st->dma_st.dma_ts), sample_count);
> +
> +	while (transferred_len >= sample_size) {
> +		iio_push_to_buffers_with_timestamp(indio_dev,
> +				(st->dma_st.rx_buf + st->dma_st.buf_idx),
> +				(st->dma_st.dma_ts + interval * sample_index));
> +		/* adjust remaining length */
> +		transferred_len -= sample_size;
> +		/* adjust buffer index */
> +		st->dma_st.buf_idx += sample_size;
> +		/* in case of reaching end of buffer, reset index */
> +		if (st->dma_st.buf_idx >= st->dma_st.rx_buf_sz)
> +			st->dma_st.buf_idx = 0;
> +		sample_index++;
> +	}
> +	/* adjust saved time for next transfer handling */
> +	st->dma_st.dma_ts = iio_get_time_ns(indio_dev);
> +}
> +
> +static irqreturn_t at91_adc_trigger_handler(int irq, void *p)
> +{
> +	struct iio_poll_func *pf = p;
> +	struct iio_dev *indio_dev = pf->indio_dev;
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +
> +	if (st->dma_st.dma_chan)
> +		at91_adc_trigger_handler_dma(indio_dev);
> +	else
> +		at91_adc_trigger_handler_nodma(indio_dev, pf);
> +
> +	iio_trigger_notify_done(indio_dev->trig);
>  
>  	return IRQ_HANDLED;
>  }
> @@ -405,7 +639,7 @@ static int at91_adc_buffer_init(struct iio_dev *indio)
>  {
>  	return devm_iio_triggered_buffer_setup(&indio->dev, indio,
>  			&iio_pollfunc_store_time,
> -			&at91_adc_trigger_handler, NULL);
> +			&at91_adc_trigger_handler, &at91_buffer_setup_ops);
>  }
>  
>  static unsigned at91_adc_startup_time(unsigned startup_time_min,
> @@ -476,10 +710,17 @@ static irqreturn_t at91_adc_interrupt(int irq, void *private)
>  	if (!(status & imr))
>  		return IRQ_NONE;
>  
> -	if (iio_buffer_enabled(indio)) {
> +	/* if we reached this point, we cannot sample faster */
> +	if (status & AT91_SAMA5D2_IER_GOVRE)
> +		pr_alert_ratelimited("Conversion overrun detected\n");
> +
> +	if (iio_buffer_enabled(indio) && !st->dma_st.dma_chan) {
>  		disable_irq_nosync(irq);
>  		iio_trigger_poll(indio->trig);
> -	} else {
> +	} else if (iio_buffer_enabled(indio) && st->dma_st.dma_chan) {
> +		disable_irq_nosync(irq);
> +		WARN(true, "Unexpected irq occurred\n");
> +	} else if (!iio_buffer_enabled(indio)) {
>  		st->conversion_value = at91_adc_readl(st, st->chan->address);
>  		st->conversion_done = true;
>  		wake_up_interruptible(&st->wq_data_available);
> @@ -571,9 +812,111 @@ static int at91_adc_write_raw(struct iio_dev *indio_dev,
>  	return 0;
>  }
>  
> +static void at91_adc_dma_init(struct platform_device *pdev)
> +{
> +	struct iio_dev *indio_dev = platform_get_drvdata(pdev);
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +	struct dma_slave_config config = {0};
> +	unsigned int pages = DIV_ROUND_UP(AT91_HWFIFO_MAX_SIZE *
> +					  AT91_BUFFER_MAX_BYTES * 2, PAGE_SIZE);
> +
> +	if (st->dma_st.dma_chan)
> +		return;
> +
> +	st->dma_st.dma_chan = dma_request_slave_channel(&pdev->dev, "rx");
> +
> +	if (!st->dma_st.dma_chan)  {
> +		dev_info(&pdev->dev, "can't get DMA channel\n");
> +		goto dma_exit;
> +	}
> +
> +	st->dma_st.rx_buf = dma_alloc_coherent(st->dma_st.dma_chan->device->dev,
> +					       pages * PAGE_SIZE,
> +					       &st->dma_st.rx_dma_buf,
> +					       GFP_KERNEL);
> +	if (!st->dma_st.rx_buf) {
> +		dev_info(&pdev->dev, "can't allocate coherent DMA area\n");
> +		goto dma_chan_disable;
> +	}
> +
> +	/* Configure DMA channel to read data register */
> +	config.direction = DMA_DEV_TO_MEM;
> +	config.src_addr = (phys_addr_t)(st->dma_st.phys_addr
> +			  + AT91_SAMA5D2_LCDR);
> +	config.src_addr_width = DMA_SLAVE_BUSWIDTH_2_BYTES;
> +	config.src_maxburst = 1;
> +	config.dst_maxburst = 1;
> +
> +	if (dmaengine_slave_config(st->dma_st.dma_chan, &config)) {
> +		dev_info(&pdev->dev, "can't configure DMA slave\n");
> +		goto dma_free_area;
> +	}
> +
> +	dev_info(&pdev->dev, "using %s for rx DMA transfers\n",
> +		 dma_chan_name(st->dma_st.dma_chan));
> +
> +	return;
> +
> +dma_free_area:
> +	dma_free_coherent(st->dma_st.dma_chan->device->dev, pages * PAGE_SIZE,
> +			  st->dma_st.rx_buf, st->dma_st.rx_dma_buf);
> +dma_chan_disable:
> +	dma_release_channel(st->dma_st.dma_chan);
> +	st->dma_st.dma_chan = 0;
> +dma_exit:
> +	dev_info(&pdev->dev, "continuing without DMA support\n");
> +}
> +
> +static void at91_adc_dma_disable(struct platform_device *pdev)
> +{
> +	struct iio_dev *indio_dev = platform_get_drvdata(pdev);
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +	unsigned int pages = DIV_ROUND_UP(AT91_HWFIFO_MAX_SIZE *
> +					  AT91_BUFFER_MAX_BYTES * 2, PAGE_SIZE);
> +
> +	/* if we are not using DMA, just return */
> +	if (!st->dma_st.dma_chan)
> +		return;
> +
> +	/* wait for all transactions to be terminated first*/
> +	dmaengine_terminate_sync(st->dma_st.dma_chan);
> +
> +	dma_free_coherent(st->dma_st.dma_chan->device->dev, pages * PAGE_SIZE,
> +			  st->dma_st.rx_buf, st->dma_st.rx_dma_buf);
> +	dma_release_channel(st->dma_st.dma_chan);
> +	st->dma_st.dma_chan = 0;
> +
> +	dev_info(&pdev->dev, "continuing without DMA support\n");
> +}
> +
> +static int at91_adc_set_watermark(struct iio_dev *indio_dev, unsigned int val)
> +{
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +
> +	if (val > AT91_HWFIFO_MAX_SIZE)
> +		return -EINVAL;
> +
> +	dev_dbg(&indio_dev->dev, "new watermark is %u\n", val);
> +	st->dma_st.watermark = val;
> +
> +	/*
> +	 * The logic here is: if we have watermark 1, it means we do
> +	 * each conversion with it's own IRQ, thus we don't need DMA.
> +	 * If the watermark is higher, we do DMA to do all the transfers in bulk
> +	 */
> +

> +	if (val == 1)
> +		at91_adc_dma_disable(to_platform_device(&indio_dev->dev));
> +	else if (val > 1)
> +		at91_adc_dma_init(to_platform_device(&indio_dev->dev));
> +
> +	return 0;
> +}
> +
>  static const struct iio_info at91_adc_info = {
>  	.read_raw = &at91_adc_read_raw,
>  	.write_raw = &at91_adc_write_raw,
> +	.hwfifo_set_watermark = &at91_adc_set_watermark,
>  	.driver_module = THIS_MODULE,
>  };
>  
> @@ -591,6 +934,42 @@ static void at91_adc_hw_init(struct at91_adc_state *st)
>  	at91_adc_setup_samp_freq(st, st->soc_info.min_sample_rate);
>  }
>  
> +static ssize_t at91_adc_get_fifo_state(struct device *dev,
> +				       struct device_attribute *attr, char *buf)
> +{
> +	struct iio_dev *indio_dev =
> +			platform_get_drvdata(to_platform_device(dev));
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +
> +	return scnprintf(buf, PAGE_SIZE, "%d\n", !!st->dma_st.dma_chan);
> +}
> +
> +static ssize_t at91_adc_get_watermark(struct device *dev,
> +				      struct device_attribute *attr, char *buf)
> +{
> +	struct iio_dev *indio_dev =
> +			platform_get_drvdata(to_platform_device(dev));
> +	struct at91_adc_state *st = iio_priv(indio_dev);
> +
> +	return scnprintf(buf, PAGE_SIZE, "%d\n", st->dma_st.watermark);
> +}
> +
> +static IIO_DEVICE_ATTR(hwfifo_enabled, 0444,
> +		       at91_adc_get_fifo_state, NULL, 0);
> +static IIO_DEVICE_ATTR(hwfifo_watermark, 0444,
> +		       at91_adc_get_watermark, NULL, 0);
> +
> +static IIO_CONST_ATTR(hwfifo_watermark_min, "2");
> +static IIO_CONST_ATTR(hwfifo_watermark_max, AT91_HWFIFO_MAX_SIZE_STR);
> +
> +static const struct attribute *at91_adc_fifo_attributes[] = {
> +	&iio_const_attr_hwfifo_watermark_min.dev_attr.attr,
> +	&iio_const_attr_hwfifo_watermark_max.dev_attr.attr,
> +	&iio_dev_attr_hwfifo_watermark.dev_attr.attr,
> +	&iio_dev_attr_hwfifo_enabled.dev_attr.attr,
> +	NULL,
> +};
> +
>  static int at91_adc_probe(struct platform_device *pdev)
>  {
>  	struct iio_dev *indio_dev;
> @@ -666,6 +1045,9 @@ static int at91_adc_probe(struct platform_device *pdev)
>  	if (!res)
>  		return -EINVAL;
>  
> +	/* if we plan to use DMA, we need the physical address of the regs */
> +	st->dma_st.phys_addr = res->start;
> +
>  	st->base = devm_ioremap_resource(&pdev->dev, res);
>  	if (IS_ERR(st->base))
>  		return PTR_ERR(st->base);
> @@ -729,18 +1111,30 @@ static int at91_adc_probe(struct platform_device *pdev)
>  		goto per_clk_disable_unprepare;
>  	}
>  
> +	if (dma_coerce_mask_and_coherent(&indio_dev->dev, DMA_BIT_MASK(32)))
> +		dev_info(&pdev->dev, "cannot set DMA mask to 32-bit\n");
> +
> +	/* the iio buffer has initially a length of 2 and a watermark of 1 */
> +	st->dma_st.watermark = 1;
> +
> +	iio_buffer_set_attrs(indio_dev->buffer, at91_adc_fifo_attributes);
> +
>  	ret = iio_device_register(indio_dev);
>  	if (ret < 0)
> -		goto per_clk_disable_unprepare;
> +		goto dma_disable;
>  
>  	dev_info(&pdev->dev, "setting up trigger as %s\n",
>  		 st->selected_trig->name);
>  
> +	dev_info(&pdev->dev, "continuing without DMA support\n");
> +
This feels a little misleading.  I think the point here is that dma
is initially disabled because fo the default watermark.

If it makes sense to put any info statement here at all it should
be more specific rather than implying we are running entirely
without DMA support.

>  	dev_info(&pdev->dev, "version: %x\n",
>  		 readl_relaxed(st->base + AT91_SAMA5D2_VERSION));
>  
>  	return 0;
>  
> +dma_disable:
> +	at91_adc_dma_disable(pdev);
>  per_clk_disable_unprepare:
>  	clk_disable_unprepare(st->per_clk);
>  vref_disable:
> @@ -757,6 +1151,8 @@ static int at91_adc_remove(struct platform_device *pdev)
>  
>  	iio_device_unregister(indio_dev);
>  
> +	at91_adc_dma_disable(pdev);
> +
>  	clk_disable_unprepare(st->per_clk);
>  
>  	regulator_disable(st->vref);