High-speed operation of certain SPI NAND/NOR controllers, such as the
Cadence OSPI controller, requires execution of a PHY tuning sequence
after switching the device to the appropriate mode (SDR or DDR).
Without PHY, the controller in SDR mode can operate only at one-fourth
of the reference clock. This series introduces an SDR-based PHY tuning
procedure to enable higher-speed operation in SDR mode.

A new _execute_tuning() hook is added to the mtd_info structure for
each MTD device. This hook is invoked when a flash partition named
'ospi.phypattern' is detected. The flash-specific _execute_tuning()
implementation retrieves the PHY tuning parameters-such as the
pre-defined tuning pattern and its size-and uses them to perform a
readback/compare operation against the known pre-flashed data in
'ospi.phypattern' as part of the tuning process.

From the flash driver, spi_mem_execute_tuning() is called with an
appropriate read_op. This function invokes cqspi_execute_tuning(),
which contains the PHY tuning algorithm. These APIs are flash-agnostic,
relying only on the correct read_op being passed. DDR mode tuning with
DQS support will be implemented in a future series.

In the NOR driver, _execute_tuning() follows the same approach of
pre-flashing a known pattern into the target partition and reading it
back during tuning. For NAND flashes, the tuning pattern can be written
directly to the cache via write_to_cache() and subsequently read back
using read_from_cache() during the tuning procedure. This support will
be added in a later phase.

At present, the series does not implement fallback handling for tuning
failures (e.g., reducing maximum frequency or adjusting dummy cycles).
Future enhancements will add fallback mechanisms to select a lower
operating frequency and optimal dummy cycle configuration.

Device Tree updates required for this functionality will be submitted
in a separate patch series after this one is merged.

This series was tested on TI's AM62A SK with W35N01JW OSPI NAND flash.
The read and write throughput with and without PHY:

Without PHY:
     READ: 7.1 MB/s
     WRITE: 6 MB/s
     
Wth PHY:
     READ: 35.1 MB/s
     WRITE: 9.2 MB/s
     
Repo: https://github.com/santhosh21/linux/commits/spinand_phy_rfc_v1
Test log: https://gist.github.com/santhosh21/d2d52cc03d2423200971b0ab9c3b9365

Signed-off-by: Santhosh Kumar K <s-k6@ti.com>

Pratyush Yadav (2):
  spi: spi-mem: Introduce support for tuning controller
  spi: cadence-quadspi: Enable PHY for aligned DAC reads

Santhosh Kumar K (8):
  spi: spi-mem: Define spi_mem_tuning_params and
    spi_mem_get_tuning_params()
  mtd: nand: spi: Introduce _execute_tuning for mtd devices
  mtd: mtdcore: Call mtd_execute_tuning during mtd_register
  spi: cadence-quadspi: Move cqspi_readdata_capture() above all
    operations
  spi: cadence-quadspi: Use BIT() macro for CQSPI_REG_READCAPTURE_BYPASS
  spi: cadence-quadspi: Enable PHY for data writes
  spi: cadence-quadspi: Implement PHY for higher frequencies in SDR mode
  spi: cadence-quadspi: Define cqspi_get_tuning_params()

 drivers/mtd/mtdcore.c             |  19 +
 drivers/mtd/mtdcore.h             |   1 +
 drivers/mtd/nand/spi/core.c       |  61 +++
 drivers/spi/spi-cadence-quadspi.c | 727 ++++++++++++++++++++++++++++--
 drivers/spi/spi-mem.c             |  22 +
 include/linux/mtd/mtd.h           |   1 +
 include/linux/spi/spi-mem.h       |  23 +
 7 files changed, 823 insertions(+), 31 deletions(-)

-- 
2.34.1