linux-kernel - Re: [PATCH 2/2] spi-atmel: support inter-word delay

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Date:   Fri, 25 Jan 2019 12:47:15 +0100
From:   Jonas Bonn <jonas@...rbonn.se>
To:     linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org
Cc:     Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Mark Brown <broonie@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] spi-atmel: support inter-word delay



On 25/01/2019 12:44, Jonas Bonn wrote:
> If the SPI slave requires an inter-word delay, configure the DLYBCT
> register accordingly.
> 
> Tested on a SAMA5D2 board (derived from SAMA5D2-Xplained reference
> board).
> 
> Signed-off-by: Jonas Bonn <jonas@...rbonn.se>
> CC: Nicolas Ferre <nicolas.ferre@...rochip.com>
> CC: Mark Brown <broonie@...nel.org>
> CC: Alexandre Belloni <alexandre.belloni@...tlin.com>
> CC: Ludovic Desroches <ludovic.desroches@...rochip.com>
> CC: linux-spi@...r.kernel.org
> CC: linux-arm-kernel@...ts.infradead.org
> ---
>   drivers/spi/spi-atmel.c | 11 ++++++-----
>   1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/spi/spi-atmel.c b/drivers/spi/spi-atmel.c
> index 74fddcd3282b..88ff3fef56e9 100644
> --- a/drivers/spi/spi-atmel.c
> +++ b/drivers/spi/spi-atmel.c
> @@ -1209,13 +1209,14 @@ static int atmel_spi_setup(struct spi_device *spi)
>   		csr |= SPI_BIT(CSAAT);
>   
>   	/* DLYBS is mostly irrelevant since we manage chipselect using GPIOs.
> -	 *
> -	 * DLYBCT would add delays between words, slowing down transfers.
> -	 * It could potentially be useful to cope with DMA bottlenecks, but
> -	 * in those cases it's probably best to just use a lower bitrate.
>   	 */
>   	csr |= SPI_BF(DLYBS, 0);
> -	csr |= SPI_BF(DLYBCT, 0);
> +
> +	/* DLYBCT adds delays between words.  This is useful for slow devices
> +	 * that need a bit of time to setup the next transfer.
> +	 */
> +	csr |= SPI_BF(DLYBCT,
> +		clamp_t(u8, (as->spi_clk/1000000*spi->word_delay)>>5, 1, 255));

Ah... this is stupid.  The clamping here _always_ introduces a delay, 
which is not what we want...  will resubmit this.

/Jonas

>   
>   	/* chipselect must have been muxed as GPIO (e.g. in board setup) */
>   	npcs_pin = (unsigned long)spi->controller_data;
>