• Lukas Wunner's avatar
    x86/quirks: Reintroduce scanning of secondary buses · 0e0c926e
    Lukas Wunner authored
    commit 850c3210 upstream.
    
    We used to scan secondary buses until the following commit that
    was applied in 2009:
    
      8659c406 ("x86: only scan the root bus in early PCI quirks")
    
    which commit constrained early quirks to the root bus only. Its
    motivation was to prevent application of the nvidia_bugs quirk
    on secondary buses.
    
    We're about to add a quirk to reset the Broadcom 4331 wireless card on
    2011/2012 Macs, which is located on a secondary bus behind a PCIe root
    port. To facilitate that, reintroduce scanning of secondary buses.
    
    The commit message of 8659c406 notes that scanning only the root bus
    "saves quite some unnecessary scanning work". The algorithm used prior
    to 8659c406 was particularly time consuming because it scanned
    buses 0 to 31 brute force. To avoid lengthening boot time, employ a
    recursive strategy which only scans buses that are actually reachable
    from the root bus.
    
    Yinghai Lu pointed out that the secondary bus number read from a
    bridge's config space may be invalid, in particular a value of 0 would
    cause an infinite loop. The PCI core goes beyond that and recurses to a
    child bus only if its bus number is greater than the parent bus number
    (see pci_scan_bridge()). Since the root bus is numbered 0, this implies
    that secondary buses may not be 0. Do the same on early scanning.
    
    If this algorithm is found to significantly impact boot time or cause
    infinite loops on broken hardware, it would be possible to limit its
    recursion depth: The Broadcom 4331 quirk applies at depth 1, all others
    at depth 0, so the bus need not be scanned deeper than that for now. An
    alternative approach would be to revert to scanning only the root bus,
    and apply the Broadcom 4331 quirk to the root ports 8086:1c12, 8086:1e12
    and 8086:1e16. Apple always positioned the card behind either of these
    three ports. The quirk would then check presence of the card in slot 0
    below the root port and do its deed.
    Signed-off-by: default avatarLukas Wunner <lukas@wunner.de>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Bjorn Helgaas <bhelgaas@google.com>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Yinghai Lu <yinghai@kernel.org>
    Cc: linux-pci@vger.kernel.org
    Link: http://lkml.kernel.org/r/f0daa70dac1a9b2483abdb31887173eb6ab77bdf.1465690253.git.lukas@wunner.deSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    0e0c926e
early-quirks.c 17.5 KB