[PATCH v5 2/12] drm/ast: Handle configuration without P2A bridge

[Joel Stanley]

On Fri, Feb 24, 2017 at 1:11 PM, Benjamin Herrenschmidt
<benh at kernel.crashing.org> wrote:
> 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: Russell Currey <ruscur at russell.cc>
> Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> --
>
> v2. [BenH]
>     - Reworked on top of Aspeed P2A patch
>     - Cleanup overall detection via a "config_mode" and log the
>       selected mode for diagnostics purposes
>     - Add a property for the SCU straps
>
> v3. [BenH]
>     - Moved the config mode detection to a separate functionn
>     - Add reading of SCU 0x40 D[12] to detect the window is
>       closed as to not trigger a bus error by just "trying".
>       (change provided by Y.C. Chen)
> v4. [BenH]
>     - Only devices with the AST2000 PCI ID have a P2A bridge
>     - Update the P2A presence test to account for VGA only
>       mode as provided by Y.C. Chen.
> v5. [BenH]
>     - Fixup prefix of OF properties based on Joel Stanley
>       review comments.

LGTM. Thanks for the explaining the mcr regs, that stuff checks out now.

Acked-by: Joel Stanley <joel at jms.id.au>

> ---
>  drivers/gpu/drm/ast/ast_drv.h  |   6 +-
>  drivers/gpu/drm/ast/ast_main.c | 264 +++++++++++++++++++++++++----------------
>  drivers/gpu/drm/ast/ast_post.c |   7 +-
>  3 files changed, 168 insertions(+), 109 deletions(-)
>