[RFC,00/13] Generic way of dealing with broken 64-bit buses
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Message ID 20210226140305.26356-1-nsaenzjulienne@suse.de
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  • Generic way of dealing with broken 64-bit buses
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Nicolas Saenz Julienne Feb. 26, 2021, 2:02 p.m. UTC
BCM2711, Raspberry Pi 4's arm64 system on chip, contains a PCIe bus that can't
handle 64-bit accesses to its MMIO address space. The issue has already been
discussed here[1], and it turns out BCM2711 isn't the only broken device in the

In most cases, the solution to this issue is to convert writeq/readq() to into
their lo_hi/hi_lo variants and the eventual introduction of some amount of
locking. But that's not good enough for every device. For example, on some
arm's SMMU configurations atomic 64-bit accesses are mandatory. This series
tries to introduce a mechanism for drivers to be able to ascertain whether or
not they are allowed to perform 64-bit accesses.

The big question is the amount of granularity needed to deal with this
(think here of distro images):

- Build-time: if a broken platform included in the image, disallow any 64-bit
  access. Drivers that need 64-bit accesses could simply bypass the check and
  hope for the best. Imposes a performance penalty on otherwise well behaving
  platforms, and features that depend on 64bit access might be disabled
  unnecessarily. It's simple to implement, yet not very generic/future proof.

- Run-time: allow/disallow 64-bit accesses based on boot time checks (i.e.
  check which platform the kernel is running on). Gets rid of all the negative
  aspects imposed to well-behaving platforms. Well-behaving buses can't coexist
  with broken ones while using all features.

- Per-device: each device has its MMIO access properties and can take decisions
  based on its local bus. That said, I'm not aware of a system that absolutely
  needs this ATM.

This series implements the third option mainly as a proof of concept.
It's my personal preference on how to deal with this. That said, my main
aim ATM is to settle on a general approach.


[1] https://lore.kernel.org/linux-arm-kernel/c188698ca0de3ed6c56a0cf7880e1578aa753077.camel@suse.de/


Nicolas Saenz Julienne (13):
  dt-bindings: Introduce 64bit-mmio-broken
  driver core: Introduce MMIO configuration
  of: device: Introduce of_mmio_configure()
  driver core: plafrom: Introduce platform_mmio_configure()
  pci: Introduce pci_mmio_configure()
  device core: Introduce dev_64bit_mmio_supported()
  arm64: Mark ARCH_MVEBU as needing broken 64bit MMIO support
  arm64: dts: marvell: armada-ap80x: Mark config-space bus as
  iommu/arm-smmu: Make use of dev_64bit_mmio_supported()
  iommu/arm-smmu-impl: Get rid of Marvell's implementation details
  arm64: Mark ARCH_BCM2835 as needing broken 64bit MMIO support
  ARM: dts: bcm2711: Mark PCIe bus as 64bit-mmio-broken
  scsi: megaraid: Make use of dev_64bit_mmio_supported()

 .../devicetree/bindings/common-properties.txt | 15 +++++++++++
 arch/Kconfig                                  |  8 ++++++
 arch/arm/boot/dts/bcm2711.dtsi                |  1 +
 arch/arm64/Kconfig.platforms                  |  2 ++
 arch/arm64/boot/dts/marvell/armada-ap80x.dtsi |  1 +
 drivers/base/dd.c                             |  6 +++++
 drivers/base/platform.c                       |  9 +++++++
 drivers/iommu/arm/arm-smmu/arm-smmu-impl.c    | 21 ---------------
 drivers/iommu/arm/arm-smmu/arm-smmu.c         |  9 +++++++
 drivers/iommu/arm/arm-smmu/arm-smmu.h         |  9 +++++--
 drivers/of/device.c                           | 19 ++++++++++++++
 drivers/pci/pci-driver.c                      | 26 +++++++++++++++++++
 drivers/scsi/megaraid/megaraid_sas_fusion.c   | 23 ++++++++--------
 include/linux/device.h                        | 20 ++++++++++++++
 include/linux/device/bus.h                    |  3 +++
 include/linux/of_device.h                     |  8 ++++++
 16 files changed, 145 insertions(+), 35 deletions(-)