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Date:   Sat, 12 Dec 2020 11:57:15 +0100
From:   Boris Brezillon <boris.brezillon@collabora.com>
To:     Sowjanya Komatineni <skomatineni@nvidia.com>
Cc:     <thierry.reding@gmail.com>, <jonathanh@nvidia.com>,
        <broonie@kernel.org>, <robh+dt@kernel.org>, <lukas@wunner.de>,
        <bbrezillon@kernel.org>, <p.yadav@ti.com>,
        <tudor.ambarus@microchip.com>, <linux-spi@vger.kernel.org>,
        <linux-tegra@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <devicetree@vger.kernel.org>
Subject: Re: [PATCH v3 5/9] spi: spi-mem: Allow masters to transfer dummy
 cycles directly by hardware
Message-ID: <20201212115715.31a8d755@collabora.com>
In-Reply-To: <1607721363-8879-6-git-send-email-skomatineni@nvidia.com>
References: <1607721363-8879-1-git-send-email-skomatineni@nvidia.com>
        <1607721363-8879-6-git-send-email-skomatineni@nvidia.com>
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On Fri, 11 Dec 2020 13:15:59 -0800
Sowjanya Komatineni <skomatineni@nvidia.com> wrote:

> This patch adds a flag SPI_MASTER_USES_HW_DUMMY_CYCLES for the controllers
> that support transfer of dummy cycles by the hardware directly.

Hm, not sure this is a good idea. I mean, if we expect regular SPI
devices to use this feature, then why not, but if it's just for
spi-mem, I'd recommend implementing a driver-specific exec_op() instead
of using the default one.

If we go for those core changes, we should at least add a
ctrl->max_dummy_cycles field so the core can fallback to regular writes
when the number of dummy cycles in the spi_mem_op exceeds what the
controller can do.

> 
> For controller with this flag set, spi-mem driver will skip dummy bytes
> transfer in the spi message.
> 
> Controller drivers can get the number of dummy cycles from spi_message.
> 
> Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
> ---
>  drivers/spi/spi-mem.c   | 18 +++++++++++-------
>  include/linux/spi/spi.h |  8 ++++++++
>  2 files changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
> index f3a3f19..38a523b 100644
> --- a/drivers/spi/spi-mem.c
> +++ b/drivers/spi/spi-mem.c
> @@ -350,13 +350,17 @@ int spi_mem_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
>  	}
>  
>  	if (op->dummy.nbytes) {
> -		memset(tmpbuf + op->addr.nbytes + 1, 0xff, op->dummy.nbytes);
> -		xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
> -		xfers[xferpos].len = op->dummy.nbytes;
> -		xfers[xferpos].tx_nbits = op->dummy.buswidth;
> -		spi_message_add_tail(&xfers[xferpos], &msg);
> -		xferpos++;
> -		totalxferlen += op->dummy.nbytes;
> +		if (ctlr->flags & SPI_MASTER_USES_HW_DUMMY_CYCLES) {
> +			msg.dummy_cycles = (op->dummy.nbytes * 8) / op->dummy.buswidth;
> +		} else {
> +			memset(tmpbuf + op->addr.nbytes + 1, 0xff, op->dummy.nbytes);
> +			xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
> +			xfers[xferpos].len = op->dummy.nbytes;
> +			xfers[xferpos].tx_nbits = op->dummy.buswidth;
> +			spi_message_add_tail(&xfers[xferpos], &msg);
> +			xferpos++;
> +			totalxferlen += op->dummy.nbytes;
> +		}
>  	}
>  
>  	if (op->data.nbytes) {
> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> index aa09fdc..2024149 100644
> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h
> @@ -512,6 +512,8 @@ struct spi_controller {
>  
>  #define SPI_MASTER_GPIO_SS		BIT(5)	/* GPIO CS must select slave */
>  
> +#define SPI_MASTER_USES_HW_DUMMY_CYCLES	BIT(6)	/* HW dummy bytes transfer */
> +
>  	/* flag indicating this is an SPI slave controller */
>  	bool			slave;
>  
> @@ -1022,6 +1024,12 @@ struct spi_message {
>  	unsigned		actual_length;
>  	int			status;
>  
> +	/*
> +	 * dummy cycles in the message transfer. This is used by the controller
> +	 * drivers supports transfer of dummy cycles directly by the hardware.
> +	 */
> +	u8			dummy_cycles;
> +
>  	/* for optional use by whatever driver currently owns the
>  	 * spi_message ...  between calls to spi_async and then later
>  	 * complete(), that's the spi_controller controller driver.