char: xillybus: Improve control of execution flow with mutexes

This commit addresses two issues by using mutexes:

(1) Add a mutex to protect the fifo_buf_order global variable. The
    purpose of this variable is avoid repeated failed calls to
    __get_free_pages() for allocating FIFO memory, when the chunk
    size was too big. However, if two drivers are initialized at the
    same time, fifo_init() may run in parallel, and fifo_buf_order
    may be reduced too much. This is a far-fetched scenario, now
    completely prevented by fifo_buf_order_mutex.

(2) setup_channels() acquires process_in_mutex to prevent
    process_bulk_in() from accessing the xillyusb_dev struct. With
    correctly working hardware, process_bulk_in() is never called while
    setup_channels() runs, because the device has no reason to send data
    in that phase. The mutex ensures that process_bulk_in() does not
    touch the members that setup_channels() alters.

    There is no similar protection for data flow in the other direction,
    because during the setup process, the only outbound data is the
    BULK endpoint used for commands, and it remains untouched after its
    initial setup.

Assisted-by: Deepseek:v4-pro Kimi:K2.6 ChatGPT:GPT-5.5 Claude:Sonnet-4.6
Assisted-by: Sashiko-0.2.5:gemini-3.1-pro-preview
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://patch.msgid.link/20261009074557.19996-2-eli.billauer@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions