SDMMC, is it necessary to send block length command before write

[Chris Rice]

Hello, we are building a project on an STM32F7, and so far it's great. Our device includes an SDcard, and we are using the HAL layer code. There is a function HAL_SD_WriteBlocks_IT which has worked fine, but I take a closer look and I see that it sends a SDMMC_CMD_SET_BLOCKLEN command (512 bytes each time), waits for the response, and then writes the specified data.

Is this necessary? Or is this an inefficiency with respect to repeated use? It seems counterintuitive that youd have to set the block length on every write. Can we make a version of this function that doesn't set the block length and use that instead? (We will experiment with this of course, but want to understand rather than trial and error.) Thanks!

Edit: here's the code by the way. I'm wondering if the "Set block size for card" part is really necessary on each write.

/**
  * @brief  Writes block(s) to a specified address in a card. The Data transfer 
  *         is managed in interrupt mode. 
  * @note   This API should be followed by a check on the card state through
  *         HAL_SD_GetCardState().
  * @note   You could also check the IT transfer process through the SD Tx 
  *         interrupt event. 
  * @param  hsd: Pointer to SD handle
  * @param  pData: Pointer to the buffer that will contain the data to transmit
  * @param  BlockAdd: Block Address where data will be written    
  * @param  NumberOfBlocks: Number of blocks to write
  * @retval HAL status
  */
HAL_StatusTypeDef HAL_SD_WriteBlocks_IT(SD_HandleTypeDef *hsd, uint8_t *pData, uint32_t BlockAdd, uint32_t NumberOfBlocks)
{
  SDMMC_DataInitTypeDef config;
  uint32_t errorstate = HAL_SD_ERROR_NONE;
  
  if(NULL == pData)
  {
    hsd->ErrorCode |= HAL_SD_ERROR_PARAM;
    return HAL_ERROR;
  }
  
  if(hsd->State == HAL_SD_STATE_READY)
  {
    hsd->ErrorCode = HAL_DMA_ERROR_NONE;
    
    if((BlockAdd + NumberOfBlocks) > (hsd->SdCard.LogBlockNbr))
    {
      hsd->ErrorCode |= HAL_SD_ERROR_ADDR_OUT_OF_RANGE;
      return HAL_ERROR;
    }
    
    hsd->State = HAL_SD_STATE_BUSY;
    
    /* Initialize data control register */
    hsd->Instance->DCTRL = 0U;
    
    hsd->pTxBuffPtr = (uint32_t *)pData;
    hsd->TxXferSize = BLOCKSIZE * NumberOfBlocks;
    
    /* Enable transfer interrupts */
    __HAL_SD_ENABLE_IT(hsd, (SDMMC_IT_DCRCFAIL | SDMMC_IT_DTIMEOUT | SDMMC_IT_TXUNDERR | SDMMC_IT_DATAEND | SDMMC_FLAG_TXFIFOHE)); 
    
    if(hsd->SdCard.CardType != CARD_SDHC_SDXC)
    {
      BlockAdd *= 512U;
    }
    
    /* Set Block Size for Card */ 
    errorstate = SDMMC_CmdBlockLength(hsd->Instance, BLOCKSIZE);
    if(errorstate != HAL_SD_ERROR_NONE)
    {
      /* Clear all the static flags */
      __HAL_SD_CLEAR_FLAG(hsd, SDMMC_STATIC_FLAGS); 
      hsd->ErrorCode |= errorstate;
      hsd->State = HAL_SD_STATE_READY;
      return HAL_ERROR;
    }
    
    /* Write Blocks in Polling mode */
    if(NumberOfBlocks > 1U)
    {
      hsd->Context = (SD_CONTEXT_WRITE_MULTIPLE_BLOCK| SD_CONTEXT_IT);
      
      /* Write Multi Block command */ 
      errorstate = SDMMC_CmdWriteMultiBlock(hsd->Instance, BlockAdd);
    }
    else
    {
      hsd->Context = (SD_CONTEXT_WRITE_SINGLE_BLOCK | SD_CONTEXT_IT);
      
      /* Write Single Block command */ 
      errorstate = SDMMC_CmdWriteSingleBlock(hsd->Instance, BlockAdd);
    }
    if(errorstate != HAL_SD_ERROR_NONE)
    {
      /* Clear all the static flags */
      __HAL_SD_CLEAR_FLAG(hsd, SDMMC_STATIC_FLAGS); 
      hsd->ErrorCode |= errorstate;
      hsd->State = HAL_SD_STATE_READY;
      return HAL_ERROR;
    }
    
    /* Configure the SD DPSM (Data Path State Machine) */ 
    config.DataTimeOut   = SDMMC_DATATIMEOUT;
    config.DataLength    = BLOCKSIZE * NumberOfBlocks;
    config.DataBlockSize = SDMMC_DATABLOCK_SIZE_512B;
    config.TransferDir   = SDMMC_TRANSFER_DIR_TO_CARD;
    config.TransferMode  = SDMMC_TRANSFER_MODE_BLOCK;
    config.DPSM          = SDMMC_DPSM_ENABLE;
    SDMMC_ConfigData(hsd->Instance, &config);
    
    return HAL_OK;
  }
  else
  {
    return HAL_BUSY;
  }
}

[Tesla DeLorean]

Generally not, but depends if it has been set to a smaller amount for other commands/requests.

There may have been specific cards ST encountered where this was necessary.

[Chris Rice]

Thanks Clive!

Are there other commands that would typically set it to a different block size? I actually thought block size was a characteristic of the card.

Nothing else is using this command, so in our case if I just call  SDMMC_CMD_SET_BLOCKLEN once after startup, then I can read and write more efficiently? Or are there things we should be mindful of?

[Tesla DeLorean]

Seem to recall it being used for the CSD read. You could optimize at a function level, by remembering the current setting and not sending subsequent requests if they are the same.

Performance is primarily derived from multi-sector reads/writes, and not doing a lot of small f_read/f_write