SAMA5 ADC: If DMA is enabled, then you should be able to configuration larger DMA transfers

arch/arm/src/sama5/Kconfig

@@ -1678,6 +1678,32 @@ config SAMA5_ADC_DMA
 		is that there will be one DMA interrupt per conversion sequence vs.
 		one interrupt per conversion.
 
+config SAMA5_ADC_DMASAMPLES
+	int "Number of DMA samples"
+	default 2
+	depends on SAMA5_ADC_DMA
+	---help---
+		If DMA is enabled, then this will specify the number of sample to
+		collect before the DMA completes.  This is actually the number of
+		triggers; the number of collected samples will be this value times
+		the number of channels that are enabled.  You should also enable the
+		sequencer if you are DMAing multiple channels.
+
+		A value of "1" would DMA one set of samples.  That is not advised.
+		In that case the processing and data transfer overhead would be
+		greater than if DMA were disabled.  A value of 2 or more should be
+		selected.
+
+		The DMA logic uses ping-pong buffers, so the total buffering
+		requirement will be
+
+			2 Buffers * Number_of_ADC_Channels * SAMA5_ADC_DMASAMPLES * sizeof(uint32_t)
+
+		So, for example, if you had 8 ADC channels and 8 triggers per DMA
+		transfer, then the total DMA buffering requirment would be:
+
+			2 * 8 * 8 * 4 = 512 bytes.
+
 config SAMA5_ADC_AUTOCALIB
 	bool "ADC auto-calibration"
 	default n

arch/arm/src/sama5/sam_adc.c

@@ -357,6 +357,19 @@
 #  undef SAMA5_ADC_UNUSED
 #endif
 
+/* Number of DMA samples to collect */
+
+#if !defined(CONFIG_SAMA5_ADC_DMA)
+#  undef  CONFIG_SAMA5_ADC_DMASAMPLES
+#  define CONFIG_SAMA5_ADC_DMASAMPLES 1
+#elif !defined(CONFIG_SAMA5_ADC_DMASAMPLES)
+#  error CONFIG_SAMA5_ADC_DMASAMPLES must be defined
+#elif CONFIG_SAMA5_ADC_DMASAMPLES < 2
+#  warning Values of ONFIG_SAMA5_ADC_DMASAMPLES < 2 are inefficient
+#endif
+
+#define SAMA5_ADC_SAMPLES (CONFIG_SAMA5_ADC_DMASAMPLES * SAMA5_NCHANNELS)
+
 /* Clocking */
 
 #if BOARD_MCK_FREQUENCY <= SAM_ADC_MAXPERCLK
@@ -405,8 +418,8 @@ struct sam_adc_s
   /* DMA sample data buffer */
 
 #ifdef CONFIG_SAMA5_ADC_DMA
-  uint32_t evenbuf[SAMA5_NCHANNELS];
-  uint32_t oddbuf[SAMA5_NCHANNELS];
+  uint32_t evenbuf[SAMA5_ADC_SAMPLES];
+  uint32_t oddbuf[SAMA5_ADC_SAMPLES];
 #endif
 #endif /* SAMA5_ADC_HAVE_CHANNELS */
 
@@ -617,11 +630,11 @@ static void sam_adc_dmadone(void *arg)
        */
 
       cp15_invalidate_dcache((uintptr_t)buffer,
-                             (uintptr_t)buffer + SAMA5_NCHANNELS * sizeof(uint32_t));
+                             (uintptr_t)buffer + SAMA5_ADC_SAMPLES * sizeof(uint32_t));
 
       /* Process each sample */
 
-      for (i = 0; i < SAMA5_NCHANNELS; i++, buffer++)
+      for (i = 0; i < SAMA5_ADC_SAMPLES; i++, buffer++)
         {
           /* Get the sample and the channel number */
 
@@ -684,7 +697,7 @@ static void sam_adc_dmacallback(DMA_HANDLE handle, void *arg, int result)
 
   sam_adc_dmasetup(priv->dma,
                    priv->odd ? (void *)priv->oddbuf : (void *)priv->evenbuf,
-                   SAMA5_NCHANNELS * sizeof(uint32_t));
+                   SAMA5_ADC_SAMPLES * sizeof(uint32_t));
 }
 #endif
 
@@ -1008,7 +1021,7 @@ static int sam_adc_setup(struct adc_dev_s *dev)
   priv->ready   = true;
   priv->enabled = false;
 
-  sam_adc_dmasetup(priv, (void *)priv->evenbuf, SAMA5_NCHANNELS);
+  sam_adc_dmasetup(priv, (void *)priv->evenbuf, SAMA5_ADC_SAMPLES);
 #else
   /* Enable end-of-conversion interrupts for all enabled channels. */