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>
---
v2 -> v3:
  - explicitly specify alignment (Jonathan)
  - increase buffer size to hold additional timestamp
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 | 39 +++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/drivers/iio/adc/stm32-adc.c b/drivers/iio/adc/stm32-adc.c
index c067c994dae2..ab2f440d7afb 100644
--- a/drivers/iio/adc/stm32-adc.c
+++ b/drivers/iio/adc/stm32-adc.c

@@ -177,7 +177,7 @@ struct stm32_adc_cfg {
  * @offset:		ADC instance register offset in ADC block
  * @cfg:		compatible configuration data
  * @completion:		end of single conversion completion
- * @buffer:		data buffer
+ * @buffer:		data buffer + 8 bytes for timestamp if enabled

  * @clk:		clock for this adc instance
  * @irq:		interrupt for this adc instance
  * @lock:		spinlock

@@ -200,7 +200,7 @@ struct stm32_adc {
 	u32			offset;
 	const struct stm32_adc_cfg	*cfg;
 	struct completion	completion;
-	u16			buffer[STM32_ADC_MAX_SQ];
+	u16			buffer[STM32_ADC_MAX_SQ + 8] __aligned(8);