Bug when mapping designware PCIe MSI msg

[William McVicker <willmcvicker@google.com>]

Hi All,

I've been debugging a PCIe dma mapping issue and I believe I have tracked the
bug down to how the designware PCIe host driver is mapping the MSI msg. In
commit 07940c369a6b ("PCI: dwc: Fix MSI page leakage in suspend/resume") [1],
the PCIe driver was re-worked to drop allocating a page for the MSI msg in
favor of using an address from the driver data. Then in commit 660c486590aa
("PCI: dwc: Set 32-bit DMA mask for MSI target address allocation") [2],
a 32-bit DMA mask was enforced for this MSI msg address in order to support
both 32-bit and 64-bit MSI address capable hardware. Both of these changes
together expose a bug on hardware that supports an MSI address greather than
32-bits. For example, the Pixel 6 supports a 36-bit MSI address and therefore
calls:

  dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(36));

Before [2], this was fine because getting an address for the driver data that
was less than or equal to 36-bits was common enough to not hit this issue, but
after [2] I started hitting the below DMA buffer overflow when the driver data
address was greater than 32-bits:

  exynos-pcie-rc 14520000.pcie: DMA addr 0x000000088536d908+2 overflow (mask ffffffff, bus limit 0).
          : WARNING: CPU: 3 PID: 8 at kernel/dma/direct.h:99 dma_map_page_attrs+0x254/0x278
  ...
  Hardware name: Oriole DVT (DT)
  Workqueue: events_unbound deferred_probe_work_func
  pstate  : 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
  pc      : dma_map_page_attrs+0x254/0x278
  lr      : dma_map_page_attrs+0x250/0x278
  sp      : ffffffc0080938b0
  ...
  Call trace:
          : dma_map_page_attrs+0x254/0x278
          : dma_map_single_attrs+0xdc/0x10c
          : dw_pcie_host_init+0x4a0/0x78c
          : exynos_pcie_rc_add_port+0x7c/0x104 [pcie_exynos_gs]
          : exynos_pcie_rc_probe+0x4c8/0x6ec [pcie_exynos_gs]
          : platform_probe+0x80/0x200
          : really_probe+0x1cc/0x458
          : __driver_probe_device+0x204/0x260
          : driver_probe_device+0x44/0x4b0
          : __device_attach_driver+0x200/0x308
          : __device_attach+0x20c/0x330


The underlying issue is that using the driver data (which can be a 64-bit
address) for the MSI msg mapping causes a DMA_MAPPING_ERROR when the dma mask
is less than 64-bits. I'm not familiar enough with the dma mapping code to
suggest a full-proof solution to solve this; however, I don't think reverting
[1] is a great solution since it addresses a valid issue and reverting [2]
doesn't actually solve the bug since the driver data address isn't restricted
by the dma mask.

I hope that helps explain the issue. Please let me know your thoughts on how we
should address this.

Thanks,
Will


[1] https://lore.kernel.org/all/20201009155505.5a580ef5@xhacker.debian/
[2] https://lore.kernel.org/r/20201117165312.25847-1-vidyas@nvidia.com