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Mike Looijmans 127b6aba32 usb: dwc3: xilinx: Prevent spike in reset signal 
commit 38d6e60b6f upstream.

The "reset" GPIO controls the RESET signal to an external, usually
ULPI PHY, chip. The original code path acquires the signal in LOW
state, and then immediately asserts it HIGH again, if the reset
signal defaulted to asserted, there'd be a short "spike" before the
reset.

Here is what happens depending on the pre-existing state of the reset
signal:
Reset (previously asserted):   ~~~|_|~~~~|_______
Reset (previously deasserted): _____|~~~~|_______
                                  ^ ^    ^
                                  A B    C

At point A, the low going transition is because the reset line is
requested using GPIOD_OUT_LOW. If the line is successfully requested,
the first thing we do is set it high _without_ any delay. This is
point B. So, a glitch occurs between A and B.

Requesting the line using GPIOD_OUT_HIGH eliminates the A and B
transitions. Instead we get:

Reset (previously asserted)  : ~~~~~~~~~~|______
Reset (previously deasserted): ____|~~~~~|______
                                   ^     ^
                                   A     C

Where A and C are the points described above in the code. Point B
has been eliminated.

The issue was found during code inspection.

Also remove the cryptic "toggle ulpi .." comment.

Fixes: ca05b38252 ("usb: dwc3: xilinx: Add gpio-reset support")
Cc: stable <stable@kernel.org>
Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20250318064518.9320-1-mike.looijmans@topic.nl
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2025-05-02 07:50:51 +02:00
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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 Restructured Text 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.