[GitHub] [incubator-nuttx] xiaoxiang781216 commented on a change in pull request #706: stm32h7:SPI Add buffers for DMA & and FIX DMA

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xiaoxiang781216 commented on a change in pull request #706: stm32h7:SPI Add buffers for DMA & and FIX DMA
URL: https://github.com/apache/incubator-nuttx/pull/706#discussion_r403010908
 
 

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 File path: arch/arm/src/stm32h7/stm32_spi.c
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 @@ -1768,23 +1840,48 @@ static void spi_exchange(FAR struct spi_dev_s *dev, FAR const void *txbuffer,
     }
 
 #ifdef CONFIG_STM32H7_DMACAPABLE
-  stm32_dmacfg_t dmacfg1;
-  stm32_dmacfg_t dmacfg2;
+  stm32_dmacfg_t dmatxcfg;
+  stm32_dmacfg_t dmarxcfg;
+
+  /* Setup DMAs */
+
+  /* If this bus uses a in driver buffers we will incur 2 copies,
+   * The copy cost is << less the non DMA transfer time and having
+   * the buffer in the driver ensures DMA can be used. This is bacause
+   * the API does not support passing the buffer extent so the only
+   * extent is buffer + the transfer size. These can sizes be less than
+   * the cache line size, and not aligned and tyicaly greater then 4
+   * bytes, which is about the break even point for the DMA IO overhead.
+   */
+
+  if (txbuffer && priv->txbuf)
+    {
+      if (nbytes > priv->buflen)
 
 Review comment:
   Is it safe to truncatethe buffer?

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