Re: [PATCH v3] drm/i915: Check HAS_PCH_NOP when install or reset dispaly IRQ

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Dec 22, 2016 at 01:18:16PM +0200, David Weinehall wrote:
> On Thu, Dec 22, 2016 at 10:52:29AM +0200, Jani Nikula wrote:
> > On Thu, 22 Dec 2016, "Wang, Elaine" <elaine.wang@intel.com> wrote:
> > >> 
> > >> On Thu, 22 Dec 2016, "Wang, Elaine" <elaine.wang@intel.com> wrote:
> > >> > Hi Jani, Ville,
> > >> >
> > >> > Any comment about the "PCH_NOP" vs "num_pipes == 0"?
> > >> >
> > >> > Thanks,
> > >> > Elaine
> > >> >> On Thu, 15 Dec 2016, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> > >> >> > On Mon, Dec 12, 2016 at 02:57:44PM +0800, Wang Elaine wrote:
> > >> >> >> From: Elaine Wang <elaine.wang@intel.com>
> > >> >> >>
> > >> >> >> Some platforms don't have display. To avoid accessing the
> > >> >> >> non-existent registers, check HAS_PCH_NOP before invoking display
> > >> >> >> IRQ install or reset function.
> > >> >> >>
> > >> >> >> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > >> >> >> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > >> >> >> Signed-off-by: Elaine Wang <elaine.wang@intel.com>
> > >> >> >> ---
> > >> >> >>  drivers/gpu/drm/i915/i915_irq.c | 10 +++++++---
> > >> >> >>  1 file changed, 7 insertions(+), 3 deletions(-)
> > >> >> >>
> > >> >> >> diff --git a/drivers/gpu/drm/i915/i915_irq.c
> > >> >> >> b/drivers/gpu/drm/i915/i915_irq.c index 0b119b9..369a038 100644
> > >> >> >> --- a/drivers/gpu/drm/i915/i915_irq.c
> > >> >> >> +++ b/drivers/gpu/drm/i915/i915_irq.c
> > >> >> >> @@ -2990,8 +2990,10 @@ static void gen8_irq_reset(struct
> > >> >> >> drm_device
> > >> >> *dev)
> > >> >> >>
> > >> >> POWER_DOMAIN_PIPE(pipe)))
> > >> >> >>  			GEN8_IRQ_RESET_NDX(DE_PIPE, pipe);
> > >> >> >>
> > >> >> >> -	GEN5_IRQ_RESET(GEN8_DE_PORT_);
> > >> >> >> -	GEN5_IRQ_RESET(GEN8_DE_MISC_);
> > >> >> >> +	if (!HAS_PCH_NOP(dev_priv)) {
> > >> >> >> +		GEN5_IRQ_RESET(GEN8_DE_PORT_);
> > >> >> >> +		GEN5_IRQ_RESET(GEN8_DE_MISC_);
> > >> >> >> +	}
> > >> >> >
> > >> >> > Hmm. These are north side registers so looking at PCH_NOP feels
> > >> >> > questionable.
> > >> >>
> > >> >> Indeed, num_pipes == 0 isn't exactly the same thing as HAS_PCH_NOP.
> > >> >>
> > >> >> Jani.
> > >> >
> > >> > I thought HAS_PCH_NOP had same meaning as num_pipes == 0 because I
> > >> saw
> > >> > following code in i915_drv.c. Is there any exception?
> > >> >
> > >> > https://cgit.freedesktop.org/drm-intel/tree/drivers/gpu/drm/i915/i915_
> > >> > drv.c?h=drm-intel-nightly#n145 static void intel_detect_pch(struct
> > >> > drm_i915_private *dev_priv) {
> > >> > 	struct pci_dev *pch = NULL;
> > >> >
> > >> > 	/* In all current cases, num_pipes is equivalent to the PCH_NOP
> > >> setting
> > >> > 	 * (which really amounts to a PCH but no South Display).
> > >> > 	 */
> > >> 
> > >> The key is in this comment; "In all current cases", where "current" is 3½ years
> > >> ago. IIRC this was written for some Xeons which did have a PCH but no
> > >> display. PCH_NOP is a kind of hack for those. Nowadays you don't always
> > >> have a PCH on gen 5+ (VLV, CHV, BXT, ...). You might have a PCH but only
> > >> need the North Display for some outputs. And I guess you might still have a
> > >> PCH but no display at all.
> > >> 
> > >> I'm just saying, we should not overload this hack to, say, cover platforms that
> > >> don't even have a PCH, or platforms that have a PCH but a functioning North
> > >> Display.
> > >> 
> > >> BR,
> > >> Jani.
> > >> 
> > > I understand your point now. Thank you for explaining this. I'll update the patch and
> > > Use num_pipes for checking whether display engine exists.
> > 
> > Ville, how about adding something like:
> > 
> > #define HAS_DISPLAY(dev_priv) (INTEL_INFO(dev_priv)->num_pipes == 0)
> > 
> > and possibly
> > 
> > #define HAS_SOUTH_DISPLAY(dev_priv) HAS_PCH_NOP(dev_priv)
> > 
> > So we could clarify the code, and abstract the rules, so it'll perhaps
> > be easier to change them later?
> > 
> > Though I fear we may end up needing to add a finer granularity depending
> > on which parts we do need to and must not touch, and that might not
> > always map to low granularity north/south display. *shrug*
> 
> Even if we might end up making things more granular later on,
> I'm a big fan of self-documenting code. HAS_DISPLAY() clearly
> explains what we're testing for, as does HAS_SOUTH_DISPLAY().

+1 on HAS_DISPLAY.

Re PCH_NOP vs. PCH_NONE:
- PCH_NONE is for platforms where there's really no PCH anywhere.
- PCH_NOP is for platforms that in general have it, but don't touch it
  it's kinda disabled. This is somewhat relevant to make sure all the
  HAS_PCH_SPLIT checks (of which not all are exclusively in display-only
  code) still work correctly for those platforms.

Given that I'm not entirely sure what you're aiming for with
HAS_SOUTH_DISPLAY ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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