[Intel-gfx] [PATCH v2] drm/i915/mcr: Hold GT forcewake during steering operations

[Andi Shyti]

Hi Matt,

On Thu, Oct 19, 2023 at 10:02:42AM -0700, Matt Roper wrote:
> The steering control and semaphore registers are inside an "always on"
> power domain with respect to RC6.  However there are some issues if
> higher-level platform sleep states are entering/exiting at the same time
> these registers are accessed.  Grabbing GT forcewake and holding it over
> the entire lock/steer/unlock cycle ensures that those sleep states have
> been fully exited before we access these registers.
> 
> This is expected to become a formally documented/numbered workaround
> soon.
> 
> Note that this patch alone isn't expected to have an immediately
> noticeable impact on MCR (mis)behavior; an upcoming pcode firmware
> update will also be necessary to provide the other half of this
> workaround.

right... I did try this, but so fare we hold the forcewake when
calling mcr_lock().

> v2:
>  - Move the forcewake inside the Xe_LPG-specific IP version check.  This
>    should only be necessary on platforms that have a steering semaphore.
> 
> Fixes: 4186e2185b4f ("drm/i915/gt: Add dedicated MCR lock")

Is this the right Fixes tag? This is adding the spinlock around
MCR, but the power domain needs to be taken independently from
the lock... I think the right fix here is

Fixes: 3100240bf846 ("drm/i915/mtl: Add hardware-level lock for steering")
Cc: <stable at vger.kernel.org> # v6.3+

When the access to the hardware was added.

BTW, given that currently we hold the forcewake already, is this
really a fix or is this more looking at the future? Is the Fixes
tag necessary?

> Cc: Radhakrishna Sripada <radhakrishna.sripada at intel.com>
> Cc: Jonathan Cavitt <jonathan.cavitt at intel.com>
> Signed-off-by: Matt Roper <matthew.d.roper at intel.com>

In any case,

Reviewed-by: Andi Shyti <andi.shyti at linux.intel.com>

Andi