Re: [PATCH] spi: tegra114: initialize native chip selects inactive

[Abraham Zukor <abe@maticrobots.com>]

On 27/07/2026 13:07, Jon Hunter wrote:
> What about the case where the SPI devices CS is active high or a
> mixture of the two?

An all-active-high bus is the case the window gets right today: probe
writes zero to the field, so an active-high CS idles deasserted and an
active-low one sits asserted. This patch inverts that. No single
constant covers a mixed board - one polarity or the other is asserted
until tegra_spi_setup() runs for that device - so the question is
which default is the right one, and active low is what the SPI core
assumes when a device says nothing.

Setting that default in probe and correcting it per device later is an
established model for controllers with a programmable CS idle level.
f9c6ef6cfe9c ("spi/xilinx: Support for spi mode CS_HIGH") added both
halves at once, to a driver that does advertise SPI_CS_HIGH:
xilinx_spi_probe() sets cs_inactive to all ones, and
xilinx_spi_setup_transfer() clears the bit for an SPI_CS_HIGH device.
The changelog gives the rationale:

  The core controls the chip select lines individually.

  By default, all the lines are consider active_low. After
  spi_setup_transfer, it has its real value.

tegra_qspi_probe() ends up in the same state by another route:
def_command1_reg = QSPI_M_S | QSPI_CS_SW_HW | QSPI_CS_SW_VAL drives
the line high in software CS mode, so its native chip select idles
deasserted before setup runs, even though its CS_POL_INACTIVE bits are
zero like ours. That is a single line on Tegra234, where cs_count is
1, so it never meets the multi-CS case, but the idle level it picks is
the active-low one.

Others leave the reset polarity alone rather than overwrite it -
sifive_spi_probe() saves and restores CSDEF, bcm63xx_hsspi_probe()
reads GLOBAL_CTRL_CS_POLARITY back into bs->cs_polarity.
tegra_spi_probe() does neither: it writes the field, and writes zeros.
The Tegra234 TRM documents all CS_POL_INACTIVE bits as one after
reset, which is the inactive level for an active-low device, so probe
is discarding a value that suits the common case for one that does
not.

Thanks,
Abe