linux-kernel - [PATCH V1] spi: Add support for Double Transfer Rate (DTR) mode

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Message-Id: <20250326083954.3338597-1-quic_msavaliy@quicinc.com>
Date: Wed, 26 Mar 2025 14:09:54 +0530
From: Mukesh Kumar Savaliya <quic_msavaliy@...cinc.com>
To: broonie@...nel.org, linux-spi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc: andersson@...nel.org, konradybcio@...nel.org,
        Mukesh Kumar Savaliya <quic_msavaliy@...cinc.com>
Subject: [PATCH V1] spi: Add support for Double Transfer Rate (DTR) mode

There is no support for protocol drivers to specify whether a transfer
should use single or dual transfer mode. As a result, the SPI controller
has no way to determine this information from the user, leading to
potential limitations in transfer capabilities.

This change introduces a new field `dtr_mode` in the `spi_transfer`
structure. The `dtr_mode` field allows protocol drivers to indicate if
Double Transfer Rate (DTR) mode is supported for a given transfer. When
`dtr_mode` is set to true, the SPI controller will use DTR mode
otherwise, it will default to single transfer mode.

The QSPI controller driver uses this flag and configures single or double
transfer rate using the controller register.

The existing spi-mem driver helps configure the DTR mode but is limited
to memory devices. There is no support available to set DTR mode for
non-memory devices, e.g., touch or any generic SPI sensor. This change
is backward compatible and doesn't break existing SPI or QSPI drivers.

Changes include:
- Addition of `dtr_mode` field in `spi_transfer` structure.
- Documentation updates to reflect the new `dtr_mode` field.

Signed-off-by: Mukesh Kumar Savaliya <quic_msavaliy@...cinc.com>
---
 include/linux/spi/spi.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 0ba5e49bace4..4e1152de82cd 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -998,6 +998,7 @@ struct spi_res {
  *	processed the word, i.e. the "pre" timestamp should be taken before
  *	transmitting the "pre" word, and the "post" timestamp after receiving
  *	transmit confirmation from the controller for the "post" word.
+ * @dtr_mode: true if supports double transfer rate.
  * @timestamped: true if the transfer has been timestamped
  * @error: Error status logged by SPI controller driver.
  *
@@ -1049,6 +1050,9 @@ struct spi_res {
  * two should both be set. User can set transfer mode with SPI_NBITS_SINGLE(1x)
  * SPI_NBITS_DUAL(2x) and SPI_NBITS_QUAD(4x) to support these three transfer.
  *
+ * User may also set dtr_mode to true to use dual transfer mode if desired. if
+ * not, default considered as single transfer mode.
+ *
  * The code that submits an spi_message (and its spi_transfers)
  * to the lower layers is responsible for managing its memory.
  * Zero-initialize every field you don't set up explicitly, to
@@ -1083,6 +1087,7 @@ struct spi_transfer {
 	unsigned	tx_nbits:4;
 	unsigned	rx_nbits:4;
 	unsigned	timestamped:1;
+	bool		dtr_mode;
 #define	SPI_NBITS_SINGLE	0x01 /* 1-bit transfer */
 #define	SPI_NBITS_DUAL		0x02 /* 2-bit transfer */
 #define	SPI_NBITS_QUAD		0x04 /* 4-bit transfer */
-- 
2.25.1