[PATCH 01/13] drm/xe: Mimic i915 behavior for non-sleeping MMIO wait

[Luca Coelho]

On Mon, 2024-10-21 at 19:27 -0300, Gustavo Sousa wrote:
> In upcoming display changes, we will modify the DMC wakelock MMIO
> waiting code to choose a non-sleeping variant implementation, because
> the wakelock is also taking in atomic context.
> 
> While xe provides an explicit parameter (namely "atomic") to prevent
> xe_mmio_wait32() from sleeping, i915 does not and implements that
> behavior when slow_timeout_ms is zero.
> 
> So, for now, let's mimic what i915 does to allow for display to use
> non-sleeping MMIO wait. In the future, we should come up with a better
> and explicit interface for this behavior in i915, at least while display
> code is not an independent entity with proper interfaces between xe and
> i915.
> 
> Signed-off-by: Gustavo Sousa <gustavo.sousa at intel.com>
> ---

Makes sense.

Reviewed-by: Luca Coelho <luciano.coelho at intel.com>

Just one question/comment below.


>  .../gpu/drm/xe/compat-i915-headers/intel_uncore.h   | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/xe/compat-i915-headers/intel_uncore.h b/drivers/gpu/drm/xe/compat-i915-headers/intel_uncore.h
> index 0382beb4035b..5a57f76c1760 100644
> --- a/drivers/gpu/drm/xe/compat-i915-headers/intel_uncore.h
> +++ b/drivers/gpu/drm/xe/compat-i915-headers/intel_uncore.h
> @@ -117,10 +117,21 @@ __intel_wait_for_register(struct intel_uncore *uncore, i915_reg_t i915_reg,
>  			  unsigned int slow_timeout_ms, u32 *out_value)
>  {
>  	struct xe_reg reg = XE_REG(i915_mmio_reg_offset(i915_reg));
> +	bool atomic;
> +
> +	/*
> +	 * FIXME: We are trying to replicate the behavior from i915 here, in
> +	 * which sleep is not performed if slow_timeout_ms == 0. This hack is
> +	 * necessary because of paths in display code that are executed in
> +	 * atomic context. Setting the atomic flag based on timeout values
> +	 * doesn't feel very robust. Ideally, we should have a proper interface
> +	 * for explicitly choosing non-sleeping behavior.

I think this is just a matter of semantics.  It would look nicer to
have a more intuitive interface, but I don't think the i915
implementation is any less robust per se.  If this behavior is
documented properly, I don't see it as a real issue.

--
Cheers,
Luca.