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Date: Wed, 28 Sep 2022 15:05:10 +0300
From: Serge Semin <fancer.lancer@...il.com>
To: Will McVicker <willmcvicker@...gle.com>
Cc: Serge Semin <fancer.lancer@...il.com>,
Jingoo Han <jingoohan1@...il.com>,
Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
Lorenzo Pieralisi <lpieralisi@...nel.org>,
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 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?
>
> 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.
>
> - 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.
-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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