[GitHub] [incubator-nuttx] jlaitine commented on a change in pull request #1147: stm32h7:stm32_spi Restores internal DMA buffer broken in 574b25 and improvements.

[GitBox <gi...@apache.org>]

jlaitine commented on a change in pull request #1147:
URL: https://github.com/apache/incubator-nuttx/pull/1147#discussion_r432484340



##########
File path: arch/arm/src/stm32h7/stm32_spi.c
##########
@@ -1955,39 +1956,53 @@ static void spi_exchange(FAR struct spi_dev_s *dev, FAR const void *txbuffer,
       return;
     }
 
-  /* Setup the DMA configs */
-
-  spi_dmatxsetup(priv, txbuffer, &txdummy, nwords, &txdmacfg);
-  spi_dmarxsetup(priv, rxbuffer, (uint16_t *)rxdummy, nwords, &rxdmacfg);
-
-#ifdef CONFIG_STM32H7_DMACAPABLE
-
-  /* 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 this bus uses the driver's DMA aligned buffers (priv->txbuf != NULL)
+   * we will incur 2 copies. However the copy cost is much less the non
+   * DMA transfer time. Having the buffer in the driver ensures DMA can be
+   * used. This is needed because the stm32_dmacapable API does not support
+   * passing the buffer extent. So the only extent calculated is buffer
+   * plus the transfer size. The sizes can be less than a cache line size,
+   * and not aligned and are typically greater then 4 bytes, which is
+   * about the break even point for the DMA IO overhead.
    */
 
   if (txbuffer && priv->txbuf)

Review comment:
       Here I would do just if (priv->txbuf) and check for txbuffer down below




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