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Hang Zhou e31194bf49 spi: bcm63xx: fix premature CS deassertion on RX-only transactions 
[ Upstream commit fd9862f726 ]

On BCM6358 (and also observed on BCM6368) the controller appears to
only generate as many SPI clocks as bytes that have been written into
the TX FIFO. For RX-only transfers the driver programs the transfer
length in SPI_MSG_CTL but does not write anything into the FIFO, so
chip select is deasserted early and the RX transfer segment is never
fully clocked in.

A concrete failing case is a three-transfer MAC address read from
SPI-NOR:
  - TX 0x03 (read command)
  - TX 3-byte address
  - RX 6 bytes (MAC)

In contrast, a two-transfer JEDEC-ID read (0x9f + 6-byte RX) works
because the driver uses prepend_len and writes dummy bytes into the
TX FIFO for the RX part.

Fix this by writing 0xff dummy bytes into the TX FIFO for RX-only
segments so that the number of bytes written to the FIFO matches the
total message length seen by the controller.

Fixes: b17de07606 ("spi/bcm63xx: work around inability to keep CS up")

Signed-off-by: Hang Zhou <929513338@qq.com>
Link: https://patch.msgid.link/tencent_7AC88FCB3076489A4A7E6C2163DF1ACF8D06@qq.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-12-07 06:24:57 +09:00
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README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.