[PATCH 08/11] drm/i915/dsb: Add i915.enable_dsb module parameter

[Ville Syrjälä]

On Wed, Jun 19, 2024 at 04:24:16PM +0300, Ville Syrjälä wrote:
> On Wed, Jun 19, 2024 at 04:11:08PM +0300, Jani Nikula wrote:
> > On Wed, 19 Jun 2024, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
> > > On Tue, Jun 18, 2024 at 02:07:56PM +0300, Jani Nikula wrote:
> > >> On Tue, 11 Jun 2024, Ville Syrjala <ville.syrjala at linux.intel.com> wrote:
> > >> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > >> >
> > >> > As we extend the use of DSB for critical pipe/plane register
> > >> > programming, it'll be nice to have an escape valve at hand,
> > >> > in case things go very poorly. To that end, add a i915.enable_dsb
> > >> > modparam by which we can force the driver to take the pure mmio
> > >> > path instead.
> > >> >
> > >> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > >> > ---
> > >> >  drivers/gpu/drm/i915/display/intel_display_params.c | 3 +++
> > >> >  drivers/gpu/drm/i915/display/intel_display_params.h | 1 +
> > >> >  drivers/gpu/drm/i915/display/intel_dsb.c            | 3 +++
> > >> >  3 files changed, 7 insertions(+)
> > >> >
> > >> > diff --git a/drivers/gpu/drm/i915/display/intel_display_params.c b/drivers/gpu/drm/i915/display/intel_display_params.c
> > >> > index aebdb7b59dbf..449a31767791 100644
> > >> > --- a/drivers/gpu/drm/i915/display/intel_display_params.c
> > >> > +++ b/drivers/gpu/drm/i915/display/intel_display_params.c
> > >> > @@ -54,6 +54,9 @@ intel_display_param_named_unsafe(enable_dc, int, 0400,
> > >> >  intel_display_param_named_unsafe(enable_dpt, bool, 0400,
> > >> >  	"Enable display page table (DPT) (default: true)");
> > >> >  
> > >> > +intel_display_param_named_unsafe(enable_dsb, bool, 0600,
> > >> > +	"Enable display state buffer (DSB) (default: true)");
> > >> 
> > >> Not much point in leaving the module param 0600, is there?
> > >
> > > It'll let you try both dsb and mmio paths at runtime without
> > > having to do a full reboot/reload.
> > 
> > I mean does any code actually look at the *module* parameter runtime?
> > It's only the initial value for the device param?
> 
> You can change it via the debugfs i915_params/* thing.

Apparently the modparam vs. debugfs permissions are specified in two
different places. This is rather confusing.

Is there no way to put them in the same place? Or can we just nuke
the permission stuff from the modparam macro entirely so it won't
end up confusing me again? Looks like there is exactly one (gem related)
modparam that uses 0600, everything else seems to be 0400.

-- 
Ville Syrjälä
Intel