[U-Boot] [PATCH V2] spi: exynos: Minimise access to SPI FIFO level

[Jagan Teki <jagannadh.teki@gmail.com>]

On 30-05-2013 10:57, Rajeshwari Shinde wrote:
> Accessing SPI registers is slow, but access to the FIFO level register
> in particular seems to be extraordinarily expensive (I measure up to
> 600ns). Perhaps it is required to synchronise with the SPI byte output
> logic which might run at 1/8th of the 40MHz SPI speed (just a guess).
>
> Reduce access to this register by filling up and emptying FIFOs
> more completely, rather than just one word each time around the inner
> loop.
>
> Since the rxfifo value will now likely be much greater that what we read
> before we fill the txfifo, we only fill the txfifo halfway. This is
> because if the txfifo is empty, but the rxfifo has data in it, then writing
> too much data to the txfifo may overflow the rxfifo as data arrives.
>
> This speeds up SPI flash reading from about 1MB/s to about 2MB/s on snow.
>
> Based on "[PATCH 0/2 V3] exynos: Support a delay after deactivate for SPI"

Please use the format i suggested on earlier patch
http://patchwork.ozlabs.org/patch/247451/

Thanks,
Jagan.
>
> Signed-off-by: Simon Glass <sjg@chromium.org>
> Signed-off-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>
> ---
> Changes in V2:
> 	- Rebased on "[PATCH 0/2 V5] spi: Enable SPI_PREAMBLE Mode"
>   drivers/spi/exynos_spi.c |   27 +++++++++++++++------------
>   1 files changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/spi/exynos_spi.c b/drivers/spi/exynos_spi.c
> index deb32bd..bcca3d6 100644
> --- a/drivers/spi/exynos_spi.c
> +++ b/drivers/spi/exynos_spi.c
> @@ -259,24 +259,27 @@ static int spi_rx_tx(struct exynos_spi_slave *spi_slave, int todo,
>
>   		/* Keep the fifos full/empty. */
>   		spi_get_fifo_levels(regs, &rx_lvl, &tx_lvl);
> -		if (tx_lvl < spi_slave->fifo_size && out_bytes) {
> +		while (tx_lvl < spi_slave->fifo_size/2 && out_bytes) {
>   			temp = txp ? *txp++ : 0xff;
>   			writel(temp, &regs->tx_data);
>   			out_bytes--;
> +			tx_lvl++;
>   		}
>   		if (rx_lvl > 0) {
> -			temp = readl(&regs->rx_data);
> -			if (spi_slave->skip_preamble) {
> -				if (temp == SPI_PREAMBLE_END_BYTE) {
> -					spi_slave->skip_preamble = 0;
> -					stopping = 0;
> +			while (rx_lvl > 0) {
> +				temp = readl(&regs->rx_data);
> +				if (spi_slave->skip_preamble) {
> +					if (temp == SPI_PREAMBLE_END_BYTE) {
> +						spi_slave->skip_preamble = 0;
> +						stopping = 0;
> +					}
> +				} else {
> +					if (rxp || stopping)
> +						*rxp++ = temp;
> +					in_bytes--;
>   				}
> -			} else {
> -				if (rxp || stopping)
> -					*rxp++ = temp;
> -				in_bytes--;
> -			}
> -			toread--;
> +				toread--;
> +				rx_lvl--;
>   		} else if (!toread) {
>   			/*
>   			 * We have run out of input data, but haven't read
>