Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target address

From: Serge Semin
Date: Thu Sep 29 2022 - 15:00:46 EST

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In reply to: William McVicker: "Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target address"
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On Thu, Sep 29, 2022 at 06:50:01PM +0000, William McVicker wrote:
> On 09/29/2022, Lorenzo Pieralisi wrote:
> > 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@xxxxxxxxxx/
> > > >
> > > > Note the reported error isn't caused by the allocation procedure, but
> > > > by the mapping procedure.
> > > >
> > > > >
> > > > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > > > Signed-off-by: Will McVicker <willmcvicker@xxxxxxxxxx>
> > > > > Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
> > > > > Acked-by: Jingoo Han <jingoohan1@xxxxxxxxx>
> > > > > ---
> > > > > .../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@xxxxxxxxxx
> >
> > from the PCI queue for v6.1 so that we can restart from a clean slate.
> >
> > Lorenzo
> >
>
> Hi Lorenzo,
>
> Based on Robin's response [1], I don't think we should change the
> implementation based on MIPS32 until we have (1) someone showing MIPS32 is
> using this driver and

This patch adds the DW PCIe controller implemented in the framework of
MIPS32 arch:
https://lore.kernel.org/linux-pci/20220822184701.25246-21-Sergey.Semin@xxxxxxxxxxxxxxxxxxxx/

> (2) that there's an actual perf regression when using
> dmam_alloc_coherent(). My patch series addresses a real issue by removing the
> dependency on ZONE_DMA32.

What about finding out what is a root cause of the performance
degradation instead of just dropping the whole standard zone support?

> Even if we did drop my patches, it won't solve
> Serge's DMA mask issues since the DW PCIe host driver will continue to
> unconditionally set the mask to 32-bits.

If you moved the DMA-mask setting to the platform driver that would
have solved my problems. I am pretty much sure the generic code
shouldn't be altering the DMA-mask if it isn't aware of the actual
device capability. In case of DW PCIe controller the AXI-bus address
width is a platform specific parameter and the generic DW PCIe code
doesn't know which width is valid.

-Sergey

>
> [1] https://lore.kernel.org/linux-pci/4dc31a63-00b1-f379-c5ac-7dc9425937f4@xxxxxxx/
>
> Thanks,
> Will
>
> > > 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
> > > > >
> > > > >
> > >

Next message: Linus Torvalds: "Re: [PATCH 3/4] proc: Point /proc/net at /proc/thread-self/net instead of /proc/self/net"
Previous message: David Hildenbrand: "Re: [RFC PATCH v2 9/9] mm: Introduce Copy-On-Write PTE table"
In reply to: William McVicker: "Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target address"
Next in thread: Lorenzo Pieralisi: "Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target address"
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