[Intel-gfx] [PATCH] drm/i915: Disable MSI for all pre-gen5

[Daniel Vetter]

On Mon, Jun 26, 2017 at 11:30:51PM +0300, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> We have pretty clear evidence that MSIs are getting lost on g4x and
> somehow the interrupt logic doesn't seem to recover from that state
> even if we try hard to clear the IIR.
> 
> Disabling IER around the normal IIR clearing in the irq handler isn't
> sufficient to avoid this, so the problem really seems to be further
> up the interrupt chain. This should guarantee that there's always
> an edge if any IIR bits are set after the interrupt handler is done,
> which should normally guarantee that the CPU interrupt is generated.
> That approach seems to work perfectly on VLV/CHV, but apparently
> not on g4x.
> 
> MSI is documented to be broken on 965gm at least. The chipset spec
> says MSI is defeatured because interrupts can be delayed or lost,
> which fits well with what we're seeing on g4x. Previously we've
> already disabled GMBUS interrupts on g4x because somehow GMBUS
> manages to raise legacy interrupts even when MSI is enabled.
> 
> Since there's such widespread MSI breakahge all over in the pre-gen5
> land let's just give up on MSI on these platforms.
> 
> Seqno reporting might be negatively affected by this since the legcy
> interrupts aren't guaranteed to be ordered with the seqno writes,
> whereas MSI interrupts may be? But an occasioanlly missed seqno
> seems like a small price to pay for generally working interrupts.
> 
> Cc: stable at vger.kernel.org
> Cc: Diego Viola <diego.viola at gmail.com>
> Tested-by: Diego Viola <diego.viola at gmail.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101261
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

One caveat was that iirc we've also seen lost irq without msi, which was
originally the reason to enable this. But that was misremembering on my
side, apparently msi was done purely to get our own irq and avoid cpu
overhead with shared irq. See:

commit ed4cb4142b242d8090d3811d5eb4abf6aa985bc8
Author: Eric Anholt <eric at anholt.net>
Date:   Tue Jul 29 12:10:39 2008 -0700

    i915: Add support for MSI and interrupt mitigation.

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> ---
>  drivers/gpu/drm/i915/i915_drv.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index e9d8e9ee51b2..9167a73f3c69 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -1141,10 +1141,12 @@ static int i915_driver_init_hw(struct drm_i915_private *dev_priv)
>  	 * and the registers being closely associated.
>  	 *
>  	 * According to chipset errata, on the 965GM, MSI interrupts may
> -	 * be lost or delayed, but we use them anyways to avoid
> -	 * stuck interrupts on some machines.
> +	 * be lost or delayed, and was defeatured. MSI interrupts seem to
> +	 * get lost on g4x as well, and interrupt delivery seems to stay
> +	 * properly dead afterwards. So we'll just disable them for all
> +	 * pre-gen5 chipsets.
>  	 */
> -	if (!IS_I945G(dev_priv) && !IS_I945GM(dev_priv)) {
> +	if (INTEL_GEN(dev_priv) >= 5) {
>  		if (pci_enable_msi(pdev) < 0)
>  			DRM_DEBUG_DRIVER("can't enable MSI");
>  	}
> -- 
> 2.13.0
> 
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch