• Russell Currey's avatar
    drm/ast: Handle configuration without P2A bridge · dbc80836
    Russell Currey authored
    commit 71f677a9 upstream.
    
    The ast driver configures a window to enable access into BMC
    memory space in order to read some configuration registers.
    
    If this window is disabled, which it can be from the BMC side,
    the ast driver can't function.
    
    Closing this window is a necessity for security if a machine's
    host side and BMC side are controlled by different parties;
    i.e. a cloud provider offering machines "bare metal".
    
    A recent patch went in to try to check if that window is open
    but it does so by trying to access the registers in question
    and testing if the result is 0xffffffff.
    
    This method will trigger a PCIe error when the window is closed
    which on some systems will be fatal (it will trigger an EEH
    for example on POWER which will take out the device).
    
    This patch improves this in two ways:
    
     - First, if the firmware has put properties in the device-tree
    containing the relevant configuration information, we use these.
    
     - Otherwise, a bit in one of the SCU scratch registers (which
    are readable via the VGA register space and writeable by the BMC)
    will indicate if the BMC has closed the window. This bit has been
    defined by Y.C Chen from Aspeed.
    
    If the window is closed and the configuration isn't available from
    the device-tree, some sane defaults are used. Those defaults are
    hopefully sufficient for standard video modes used on a server.
    Signed-off-by: default avatarRussell Currey <ruscur@russell.cc>
    Acked-by: default avatarJoel Stanley <joel@jms.id.au>
    Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
    Signed-off-by: default avatarDave Airlie <airlied@redhat.com>
    Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    dbc80836
ast_main.c 15.4 KB