RE: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge systems

["Deucher, Alexander" <Alexander.Deucher@amd.com>]

> -----Original Message-----
> From: Deucher, Alexander
> Sent: Monday, December 2, 2019 11:37 AM
> To: Lucas Stach <dev@lynxeye.de>; Kai-Heng Feng
> <kai.heng.feng@canonical.com>; joro@8bytes.org; Koenig, Christian
> (Christian.Koenig@amd.com) <Christian.Koenig@amd.com>
> Cc: iommu@lists.linux-foundation.org; linux-kernel@vger.kernel.org
> Subject: RE: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge
> systems
> 
> > -----Original Message-----
> > From: Lucas Stach <dev@lynxeye.de>
> > Sent: Sunday, December 1, 2019 7:43 AM
> > To: Kai-Heng Feng <kai.heng.feng@canonical.com>; joro@8bytes.org
> > Cc: Deucher, Alexander <Alexander.Deucher@amd.com>;
> > iommu@lists.linux-foundation.org; linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge
> > systems
> >
> > Am Freitag, den 29.11.2019, 22:21 +0800 schrieb Kai-Heng Feng:
> > > Serious screen flickering when Stoney Ridge outputs to a 4K monitor.
> > >
> > > According to Alex Deucher, IOMMU isn't enabled on Windows, so let's
> > > do the same here to avoid screen flickering on 4K monitor.
> >
> > This doesn't seem like a good solution, especially if there isn't a
> > method for the user to opt-out.  Some users might prefer having the
> > IOMMU support to 4K display output.
> >
> > But before using the big hammer of disabling or breaking one of those
> > features, we should take a look at what's the issue here. Screen
> > flickering caused by the IOMMU being active hints to the IOMMU not
> > being able to sustain the translation bandwidth required by the high-
> > bandwidth isochronous transfers caused by 4K scanout, most likely due
> > to insufficient TLB space.
> >
> > As far as I know the framebuffer memory for the display buffers is
> > located in stolen RAM, and thus contigous in memory. I don't know the
> > details of the GPU integration on those APUs, but maybe there even is
> > a way to bypass the IOMMU for the stolen VRAM regions?
> >
> > If there isn't and all GPU traffic passes through the IOMMU when
> > active, we should check if the stolen RAM is mapped with hugepages on
> > the IOMMU side. All the stolen RAM can most likely be mapped with a
> > few hugepage mappings, which should reduce IOMMU TLB demand by a
> large margin.
> 
> The is no issue when we scan out of the carve out region.  The issue occurs
> when we scan out of regular system memory (scatter/gather).  Many newer
> laptops have very small carve out regions (e.g., 32 MB), so we have to use
> regular system pages to support multiple high resolution displays.  The
> problem is, the latency gets too high at some point when the IOMMU is
> involved.  Huge pages would probably help in this case, but I'm not sure if
> there is any way to guarantee that we get huge pages for system memory.  I
> guess we could use CMA or something like that.

Thomas recently sent out a patch set to add huge page support to ttm:
https://patchwork.freedesktop.org/series/70090/
We'd still need a way to guarantee huge pages for the display buffer.

Alex

> 
> Alex
> 
> >
> > Regards,
> > Lucas
> >
> > > Cc: Alex Deucher <alexander.deucher@amd.com>
> > > Bug:
> > >
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgi
> > > tl
> > >
> >
> ab.freedesktop.org%2Fdrm%2Famd%2Fissues%2F961&amp;data=02%7C01%
> > 7Calexa
> > >
> >
> nder.deucher%40amd.com%7C30540b2bf2be417c4d9508d7765bf07f%7C3dd
> > 8961fe4
> > >
> >
> 884e608e11a82d994e183d%7C0%7C0%7C637108010075463266&amp;sdata=1
> > ZIZUWos
> > > cPiB4auOY10jlGzoFeWszYMDBQG0CtrrOO8%3D&amp;reserved=0
> > > Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
> > > ---
> > > v2:
> > > - Find Stoney graphics instead of host bridge.
> > >
> > >  drivers/iommu/amd_iommu_init.c | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/iommu/amd_iommu_init.c
> > > b/drivers/iommu/amd_iommu_init.c index 568c52317757..139aa6fdadda
> > > 100644
> > > --- a/drivers/iommu/amd_iommu_init.c
> > > +++ b/drivers/iommu/amd_iommu_init.c
> > > @@ -2516,6 +2516,7 @@ static int __init early_amd_iommu_init(void)
> > >  	struct acpi_table_header *ivrs_base;
> > >  	acpi_status status;
> > >  	int i, remap_cache_sz, ret = 0;
> > > +	u32 pci_id;
> > >
> > >  	if (!amd_iommu_detected)
> > >  		return -ENODEV;
> > > @@ -2603,6 +2604,16 @@ static int __init early_amd_iommu_init(void)
> > >  	if (ret)
> > >  		goto out;
> > >
> > > +	/* Disable IOMMU if there's Stoney Ridge graphics */
> > > +	for (i = 0; i < 32; i++) {
> > > +		pci_id = read_pci_config(0, i, 0, 0);
> > > +		if ((pci_id & 0xffff) == 0x1002 && (pci_id >> 16) == 0x98e4) {
> > > +			pr_info("Disable IOMMU on Stoney Ridge\n");
> > > +			amd_iommu_disabled = true;
> > > +			break;
> > > +		}
> > > +	}
> > > +
> > >  	/* Disable any previously enabled IOMMUs */
> > >  	if (!is_kdump_kernel() || amd_iommu_disabled)
> > >  		disable_iommus();
> > > @@ -2711,7 +2722,7 @@ static int __init state_next(void)
> > >  		ret = early_amd_iommu_init();
> > >  		init_state = ret ? IOMMU_INIT_ERROR :
> > IOMMU_ACPI_FINISHED;
> > >  		if (init_state == IOMMU_ACPI_FINISHED &&
> > amd_iommu_disabled) {
> > > -			pr_info("AMD IOMMU disabled on kernel command-
> > line\n");
> > > +			pr_info("AMD IOMMU disabled\n");
> > >  			init_state = IOMMU_CMDLINE_DISABLED;
> > >  			ret = -EINVAL;
> > >  		}

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