> -----Original Message----- > From: Deucher, Alexander > Sent: Monday, December 2, 2019 11:37 AM > To: Lucas Stach <d...@lynxeye.de>; Kai-Heng Feng > <kai.heng.f...@canonical.com>; j...@8bytes.org; Koenig, Christian > (christian.koe...@amd.com) <christian.koe...@amd.com> > Cc: iommu@lists.linux-foundation.org; linux-ker...@vger.kernel.org > Subject: RE: [PATCH v2] iommu/amd: Disable IOMMU on Stoney Ridge > systems > > > -----Original Message----- > > From: Lucas Stach <d...@lynxeye.de> > > Sent: Sunday, December 1, 2019 7:43 AM > > To: Kai-Heng Feng <kai.heng.f...@canonical.com>; j...@8bytes.org > > Cc: Deucher, Alexander <alexander.deuc...@amd.com>; > > iommu@lists.linux-foundation.org; linux-ker...@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.deuc...@amd.com> > > > Bug: > > > > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgi > > > tl > > > > > > ab.freedesktop.org%2Fdrm%2Famd%2Fissues%2F961&data=02%7C01% > > 7Calexa > > > > > > nder.deucher%40amd.com%7C30540b2bf2be417c4d9508d7765bf07f%7C3dd > > 8961fe4 > > > > > > 884e608e11a82d994e183d%7C0%7C0%7C637108010075463266&sdata=1 > > ZIZUWos > > > cPiB4auOY10jlGzoFeWszYMDBQG0CtrrOO8%3D&reserved=0 > > > Signed-off-by: Kai-Heng Feng <kai.heng.f...@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; > > > } _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu