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Date: Thu, 29 Sep 2022 10:13:15 +0200
From: Lorenzo Pieralisi <lpieralisi@...nel.org>
To: William McVicker <willmcvicker@...gle.com>
Cc: Serge Semin <fancer.lancer@...il.com>,
Jingoo Han <jingoohan1@...il.com>,
Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
Rob Herring <robh@...nel.org>,
Krzysztof Wilczyński <kw@...ux.com>,
Bjorn Helgaas <bhelgaas@...gle.com>, kernel-team@...roid.com,
Vidya Sagar <vidyas@...dia.com>,
Christoph Hellwig <hch@...radead.org>,
Robin Murphy <robin.murphy@....com>, linux-pci@...r.kernel.org,
linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target
address
On Wed, Sep 28, 2022 at 05:52:26PM +0000, William McVicker wrote:
> On 09/28/2022, Serge Semin wrote:
> > On Thu, Aug 25, 2022 at 06:50:25PM +0000, Will McVicker wrote:
> > > Since not all devices require a 32-bit MSI address, add support to the
> > > PCIe host driver to allow setting the DMA mask to 64-bits if the 32-bit
> > > allocation fails. This allows kernels to disable ZONE_DMA32 and bounce
> > > buffering (swiotlb) without risking not being able to get a 32-bit address
> > > during DMA allocation.
> >
> > What is a problem in having the ZONE_DMA32 enabled anyway?
>
> On Android most devices don't have a 32-bit limitation. Several Android OEMs
> have reported significant enough performance improvements after disabling
> ZONE_DMA32. These include reducing memory usage, improving the time spent by
> kswapd, improving direct reclaim, and improving app launch time.
>
> So this patch series was introduced to remove the dependency on ZONE_DMA32 for
> the DW PCIe drivers.
>
> >
> > >
> > > Basically, in the slim chance that there are no 32-bit allocations
> > > available, the current PCIe host driver will fail to allocate the msi_msg
> > > page due to a DMA address overflow (seen in [1]). With this patch, the
> > > PCIe host can retry the allocation with a 64-bit DMA mask if the current
> > > PCIe device advertises 64-bit support via its MSI capabilities.
> > >
> > > [1] https://lore.kernel.org/all/Yo0soniFborDl7+C@google.com/
> >
> > Note the reported error isn't caused by the allocation procedure, but
> > by the mapping procedure.
> >
> > >
> > > Reported-by: kernel test robot <lkp@...el.com>
> > > Signed-off-by: Will McVicker <willmcvicker@...gle.com>
> > > Reviewed-by: Rob Herring <robh@...nel.org>
> > > Acked-by: Jingoo Han <jingoohan1@...il.com>
> > > ---
> > > .../pci/controller/dwc/pcie-designware-host.c | 38 ++++++++++++++-----
> > > drivers/pci/controller/dwc/pcie-designware.c | 8 ++++
> > > drivers/pci/controller/dwc/pcie-designware.h | 1 +
> > > 3 files changed, 38 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > index 39f3b37d4033..8928a9a29d58 100644
> > > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > @@ -330,6 +330,9 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
> > > u64 *msi_vaddr;
> > > int ret;
> > > u32 ctrl, num_ctrls;
> > > + bool msi_64bit = false;
> > > + bool retry_64bit = false;
> > > + u16 msi_capabilities;
> > >
> > > for (ctrl = 0; ctrl < MAX_MSI_CTRLS; ctrl++)
> > > pp->irq_mask[ctrl] = ~0;
> > > @@ -367,16 +370,33 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
> > > dw_chained_msi_isr, pp);
> > > }
> > >
> > > - ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
> > > - if (ret)
> > > - dev_warn(dev, "Failed to set DMA mask to 32-bit. Devices with only 32-bit MSI support may not work properly\n");
> >
> > > + msi_capabilities = dw_pcie_msi_capabilities(pci);
> > > + if (msi_capabilities & PCI_MSI_FLAGS_ENABLE)
> > > + msi_64bit = msi_capabilities & PCI_MSI_FLAGS_64BIT;
> >
> > Note this capability flag has nothing to do with the DW PCIe iMSI-RX
> > engine, which is used here to detect and report MSI TLPs. By design
> > iMSI-RX always support 64-bit addresses. If you imply having that flag
> > set by the DW PCIe platform drivers on the platform-specific probe
> > stage as an indication of MSI address range, then ok.
>
> Right. The DW PCIe device driver can set this flag during probe before calling
> dw_pcie_host init() to ensure that we will always successfully allocate and map
> the MSI target address (as required to return successfully from
> dw_pcie_host_init()).
>
> >
> > >
> > > - msi_vaddr = dmam_alloc_coherent(dev, sizeof(u64), &pp->msi_data,
> > > - GFP_KERNEL);
> > > - if (!msi_vaddr) {
> > > - dev_err(dev, "Failed to alloc and map MSI data\n");
> > > - dw_pcie_free_msi(pp);
> > > - return -ENOMEM;
> > > + while (true) {
> > > + dev_dbg(dev, "Setting MSI DMA mask to %s-bit.\n",
> > > + retry_64bit ? "64" : "32");
> >
> > > + ret = dma_set_mask_and_coherent(dev, retry_64bit ?
> > > + DMA_BIT_MASK(64) :
> > > + DMA_BIT_MASK(32));
> >
> > I'd suggest to just drop this. No DMA actually performed on getting the
> > MSI TLPs. So modifying the device DMA-mask due to something which
> > doesn't cause DMA and based on the flag which doesn't indicates the
> > device DMA-capability is at least inappropriate.
> >
> > > + if (ret)
> > > + dev_warn(dev, "Failed to set DMA mask to %s-bit.\n",
> > > + retry_64bit ? "64" : "32");
> > > +
> >
> > > + msi_vaddr = dmam_alloc_coherent(dev, sizeof(u64), &pp->msi_data,
> > > + GFP_KERNEL);
> >
> > As I noted earlier the DMA-coherent memory can be too expensive. So
> > it's a waste of one allocating with no intent of usage. Instead of this
> > just get back the alloc_page() method here and pass the flag GFP_DMA32
> > to that function if MSI-capability reported the PCI_MSI_FLAGS_64BIT
> > unset.
>
> As mentioned above, we don't want to force this driver to require the kernel to
> enable ZONE_DMA32. Since no I/O happens to this buffer, could we use
> dma_alloc_attrs() with the DMA_ATTR_SKIP_CPU_SYNC and
> DMA_ATTR_NO_KERNEL_MAPPING attribute? Would that address the "too expensive"
> issues you're referring to?
>
> With regards to the DMA mask, I'm okay with moving that out of the host
> controller and into the DW PCIe device driver. That would address all of my
> issues and we could just drop the logic for checking the PCI_MSI_FLAGS_64BIT.
> However, I'm not the one you to convince to do that.
We are late -rc7 and it does not look like we are converging on this
discussion - I will wait till tomorrow but then I will have to drop
https://lore.kernel.org/linux-pci/20220825235404.4132818-1-willmcvicker@google.com
from the PCI queue for v6.1 so that we can restart from a clean slate.
Lorenzo
> Regards,
> Will
>
> >
> > -Sergey
> >
> > > + if (!msi_vaddr) {
> > > + dev_err(dev, "Failed to alloc and map MSI data\n");
> > > + if (msi_64bit && !retry_64bit) {
> > > + retry_64bit = true;
> > > + continue;
> > > + }
> > > +
> > > + dw_pcie_free_msi(pp);
> > > + return -ENOMEM;
> > > + }
> > > + break;
> > > }
> > >
> > > return 0;
> > > diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
> > > index c6725c519a47..650a7f22f9d0 100644
> > > --- a/drivers/pci/controller/dwc/pcie-designware.c
> > > +++ b/drivers/pci/controller/dwc/pcie-designware.c
> > > @@ -82,6 +82,14 @@ u8 dw_pcie_find_capability(struct dw_pcie *pci, u8 cap)
> > > }
> > > EXPORT_SYMBOL_GPL(dw_pcie_find_capability);
> > >
> > > +u16 dw_pcie_msi_capabilities(struct dw_pcie *pci)
> > > +{
> > > + u8 offset;
> > > +
> > > + offset = dw_pcie_find_capability(pci, PCI_CAP_ID_MSI);
> > > + return dw_pcie_readw_dbi(pci, offset + PCI_MSI_FLAGS);
> > > +}
> > > +
> > > static u16 dw_pcie_find_next_ext_capability(struct dw_pcie *pci, u16 start,
> > > u8 cap)
> > > {
> > > diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
> > > index a871ae7eb59e..45fcdfc8c035 100644
> > > --- a/drivers/pci/controller/dwc/pcie-designware.h
> > > +++ b/drivers/pci/controller/dwc/pcie-designware.h
> > > @@ -332,6 +332,7 @@ void dw_pcie_version_detect(struct dw_pcie *pci);
> > >
> > > u8 dw_pcie_find_capability(struct dw_pcie *pci, u8 cap);
> > > u16 dw_pcie_find_ext_capability(struct dw_pcie *pci, u8 cap);
> > > +u16 dw_pcie_msi_capabilities(struct dw_pcie *pci);
> > >
> > > int dw_pcie_read(void __iomem *addr, int size, u32 *val);
> > > int dw_pcie_write(void __iomem *addr, int size, u32 val);
> > > --
> > > 2.37.2.672.g94769d06f0-goog
> > >
> > >
>
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