Re: [PATCH v1 1/4] spi: dw: Add DDR (Dual Data-Rate) support for enhanced SPI mode

From: Miquel Raynal <miquel.raynal@bootlin.com>
To: Changhuang Liang <changhuang.liang@starfivetech.com>
Cc: Pratyush Yadav <pratyush@kernel.org>,
	 Michael Walle <mwalle@kernel.org>,
	 Takahiro Kuwano <takahiro.kuwano@infineon.com>,
	Mark Brown <broonie@kernel.org>,
	 Richard Weinberger <richard@nod.at>,
	Vignesh Raghavendra <vigneshr@ti.com>,
	 linux-spi@vger.kernel.org, linux-kernel@vger.kernel.org,
	 linux-mtd@lists.infradead.org
Subject: Re: [PATCH v1 1/4] spi: dw: Add DDR (Dual Data-Rate) support for enhanced SPI mode
Date: Thu, 22 Oct 2026 12:41:52 +0200	[thread overview]
Message-ID: <87bj8mqaof.fsf@bootlin.com> (raw)
In-Reply-To: <20261022102614.75696-2-changhuang.liang@starfivetech.com> (Changhuang Liang's message of "Thu, 22 Oct 2026 03:26:11 -0700")

On 22/10/2026 at 03:26:11 -07, Changhuang Liang <changhuang.liang@starfivetech.com> wrote:

> Add support for DDR (Dual Data-Rate) transfers in the enhanced SPI
> mode of the DW SPI controller.
>
> The enhanced SPI control register (SPI_CTRLR0) has dedicated fields to
> enable DDR mode for the instruction phase (INST_DDR_EN) and for the
> address/dummy/data phases (SPI_DDR_EN). Populate these from the
> corresponding dtr flags of the spi_mem_op.
>
> For DDR transfers, the dummy phase consumes two bits per clock per
> line, so the computed wait cycles must be halved. Factor the wait cycle
> calculation into a helper, dw_spi_enh_wait_cycles(), so that both the
> supports check and the exec path use the same value.
>
> When SPI DDR is enabled, program the DDR_DRIVE_EDGE register. The
> recommended TDE value is clk_div / 4, clamped to clk_div / 2 - 1 and to
> the field width.
>
> Finally, advertise DTR support to the SPI memory layer through a
> dedicated dw_spi_enh_mem_caps, selected when the controller has
> DW_SPI_CAP_EMODE, so that spi-mem may issue DDR operations.
>
> Signed-off-by: Changhuang Liang <changhuang.liang@starfivetech.com>
> ---
>  drivers/spi/spi-dw-core.c | 52 +++++++++++++++++++++++++++++++++------
>  drivers/spi/spi-dw.h      |  6 +++++
>  2 files changed, 51 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/spi/spi-dw-core.c b/drivers/spi/spi-dw-core.c
> index 47434fa78e4b..6ff142c8a6bd 100644
> --- a/drivers/spi/spi-dw-core.c
> +++ b/drivers/spi/spi-dw-core.c
> @@ -46,6 +46,8 @@ struct dw_spi_enh_cfg {
>  	u8 inst_l;
>  	u8 addr_l;
>  	u8 trans_t;
> +	bool inst_ddr;
> +	bool spi_ddr;

If I understand this correctly, spi_ddr is quite a bad name because it
means address and dummy and data ddr (but not instr). Can you try to
come up with a better name for this variable?

Also, if my above understanding is correct, you must provide a
supports_op() callback which would refuse any xS-xS-xD but allow
xD-xD-xD and xS-xD-D modes.

Thanks,
Miquèl

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