[PATCH v1 5/8] spi: dw: Speed up transfer loops

[Marek Vasut]

On 3/5/20 8:19 PM, Sean Anderson wrote:
> The transfer loops are very tight on some platforms (especially on higher
> speeds). If we don't read/write fast enough we can run into over-/under-
> flow problems. This patch removes several divisions and log statements,
> and simplifies the read logic.
> 
> Signed-off-by: Sean Anderson <seanga2 at gmail.com>
> ---
> 
>  drivers/spi/designware_spi.c | 29 +++++++++--------------------
>  1 file changed, 9 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/spi/designware_spi.c b/drivers/spi/designware_spi.c
> index 38c24fe550..613eb0d0e6 100644
> --- a/drivers/spi/designware_spi.c
> +++ b/drivers/spi/designware_spi.c
> @@ -304,7 +304,7 @@ static inline u32 tx_max(struct dw_spi_priv *priv)
>  {
>  	u32 tx_left, tx_room, rxtx_gap;
>  
> -	tx_left = (priv->tx_end - priv->tx) / (priv->bits_per_word >> 3);
> +	tx_left = priv->tx_end - priv->tx;
>  	tx_room = priv->fifo_len - dw_read(priv, DW_SPI_TXFLR);
>  
>  	/*
> @@ -315,8 +315,7 @@ static inline u32 tx_max(struct dw_spi_priv *priv)
>  	 * shift registers. So a control from sw point of
>  	 * view is taken.
>  	 */
> -	rxtx_gap = ((priv->rx_end - priv->rx) - (priv->tx_end - priv->tx)) /
> -		(priv->bits_per_word >> 3);
> +	rxtx_gap = ((priv->rx_end - priv->rx) - (priv->tx_end - priv->tx));
>  
>  	return min3(tx_left, tx_room, (u32)(priv->fifo_len - rxtx_gap));
>  }
> @@ -324,7 +323,7 @@ static inline u32 tx_max(struct dw_spi_priv *priv)
>  /* Return the max entries we should read out of rx fifo */
>  static inline u32 rx_max(struct dw_spi_priv *priv)
>  {
> -	u32 rx_left = (priv->rx_end - priv->rx) / (priv->bits_per_word >> 3);
> +	u32 rx_left = priv->rx_end - priv->rx;
>  
>  	return min_t(u32, rx_left, dw_read(priv, DW_SPI_RXFLR));
>  }
> @@ -336,15 +335,10 @@ static void dw_writer(struct dw_spi_priv *priv)
>  
>  	while (max--) {
>  		/* Set the tx word if the transfer's original "tx" is not null */
> -		if (priv->tx_end - priv->len) {
> -			if (priv->bits_per_word == 8)
> -				txw = *(u8 *)(priv->tx);
> -			else
> -				txw = *(u16 *)(priv->tx);
> -		}
> +		if (priv->tx_end - priv->len)
> +			txw = *(u8 *)(priv->tx);
>  		dw_write(priv, DW_SPI_DR, txw);
> -		debug("%s: tx=0x%02x\n", __func__, txw);
> -		priv->tx += priv->bits_per_word >> 3;
> +		priv->tx++;

This breaks 16 bits per word transfers, NAK.

The compiler should be able to figure out the rest of the optimizations
in this patch.