Re: [PATCH v6 0/2] PCI: dwc: Add support for 64-bit MSI target addresses

[Serge Semin]

Hi Evgenii

On Wed, Feb 01, 2023 at 04:54:55PM +0300, Evgenii Shatokhin wrote:
> On 31.01.2023 15:42, Robin Murphy wrote:
> > 
> > On 2023-01-31 12:29, Evgenii Shatokhin wrote:
> > > Hi,
> > > 
> > > On 26.08.2022 02:54, Will McVicker wrote:
> > > > Hi All,
> > > > 
> > > > I've update patch 2/2 to address Robin's suggestions. This includes:
> > > > 
> > > >   * Dropping the while-loop for retrying with a 64-bit mask in favor of
> > > >     retrying within the error if-statement.
> > > >   * Using an int for the DMA mask instead of a bool and ternary
> > > > operation.
> > > > 
> > > > Thanks again for the reviews and sorry for the extra revision today!
> > > > Hopefully this is the last one :) If not, I'd be fine to submit
> > > > patch 1/2
> > > > without 2/2 to avoid resending patch 1/2 for future revisions of patch
> > > > 2/2
> > > > (unless I don't need to do that anyway).
> > > 
> > > The first patch of the series made it into the mainline kernel, but, it
> > > seems, the second one ("PCI: dwc: Add support for 64-bit MSI target
> > > address") did not. As of 6.2-rc6, it is still missing.
> > > 
> > > Was it intentionally dropped because of some issues or, perhaps, just by
> > > accident? If it was by accident, could you please queue it for inclusion
> > > into mainline again?
> > 
> > Yes, it was dropped due to the PCI_MSI_FLAGS_64BIT usage apparently
> > being incorrect, and some other open debate (which all happened on the
> > v5 thread):
> > 
> > https://lore.kernel.org/linux-pci/YzVTmy9MWh+AjshC@lpieralisi/
> 

> I see. If I understand it correctly, the problem was that
> PCI_MSI_FLAGS_64BIT flag did not guarantee that 64-bit mask could be used
> for that particular allocation. Right?
> 

William was trying to utilize for only software cause. Setting
PCI_MSI_FLAGS_64BIT didn't actually change the hardware behavior.
He could have as well provided just a driver private capability
flag. (see below for a more detailed problem description) 

> > 
> > The DMA mask issues have now been sorted out,
> 
> I suppose, you mean https://lore.kernel.org/all/20230113171409.30470-26-Sergey.Semin@xxxxxxxxxxxxxxxxxxxx/?

Well, the way the DMA-mask issue has been solved was a bit of the
hacky. I wouldn't call it a fully proper solution. The problem with
pointlessly allocating physical memory for the iMSI-RX engine (it
doesn't perform any DMA) and artificially restricting the coherent-DMA
mask is still there. The patch in the subject was a compromise in
order to at least permit unrestricted streaming DMAs but limiting the
coherent DMAs for the MSI setup to work properly for all peripheral
devices.

> 
> It still breaks our particular case when the SoC has no 32-bit-addressable
> RAM. We'd set DMA masks to DMA_BIT_MASK(36) in the platform-specific driver
> before calling dw_pcie_host_init(). However, dw_pcie_msi_host_init() resets
> it to 32-bit, tries dmam_alloc_coherent() and fails.

Yeah. That's another problem with the implemented approach. But are
your sure the driver had worked even before this patch? AFAICS the
driver allocated the MSI-targeted page from DMA32 zone before this
modification. So the allocation must have failed on your platform too.

> 
> With 36-bit masks, the kernel seems to play well with the devices in our
> case.
> 
> I saw your comment in https://lore.kernel.org/linux-pci/4dc31a63-00b1-f379-c5ac-7dc9425937f4@xxxxxxx/
> that drivers should always explicitly set their masks.
> 

> Is it a really bad idea to check the current coherent mask's bits in
> dw_pcie_msi_host_init() and if it is more than 32 - just issue a warning
> rather than reset it to 32-bit unconditionally? That would help in our case.
> Or, perhaps, there is a better workaround.

The problem isn't in the value the mask is set to. The problem is
two-leveled, but is mainly connected with the PCIe device detected on
the PCIe bus. There are some of them which can't send MSI TLPs to the
64-bit addresses. Since we can't predict whether such devices exist on
the bus beforehand the LLDD probe is performed together with the
MSI-engine initialization, the solution was to just restrict the MSIs
base address to be allocated within the lowest 4GB. Moreover as I said
above the iMSI-RX engine doesn't actually cause any DMA thus there is
no need in any memory allocation. Instead reserving some PCIe-bus
space/DWORD for MSIs would be enough. Alas the PCIe-subsystem doesn't
provide a way to do so. That's why we have what you see in the driver:
DMA mask restriction and coherent DMA memory allocation.

If only we had a way to auto-detected the PCIe-bus space with no
physical memory behind it and take out a DWORD from it to initialize
the iMSI-RX engine we could have immediately got rid from the mask
setting operation and the memory allocation. It would have solved your
problem too.

-Serge(y)

> 
> Looking forward to your comments.



> 
> 
> > so you, or Will, or anyone
> > else interested should be free to rework this on top of linux-next
> > (although at this point, more realistically on top of 6.3-rc1 in a few
> > weeks).
> > 
> > Thanks,
> > Robin.
> > 
> > > Support for 64-bit MSI target addresses is needed for some of our SoCs.
> > > I ran into a situation when there was no available RAM in ZONE_DMA32
> > > during initialization of PCIe host. Hence, dmam_alloc_coherent() failed
> > > in dw_pcie_msi_host_init() and initialization failed with -ENOMEM:
> > > 
> > > [    0.374834] dw-pcie 4000000.pcie0: host bridge /soc/pcie0@4000000
> > > ranges:
> > > [    0.375813] dw-pcie 4000000.pcie0:      MEM
> > > 0x0041000000..0x004fffffff -> 0x0041000000
> > > [    0.376171] dw-pcie 4000000.pcie0:   IB MEM
> > > 0x0400000000..0x07ffffffff -> 0x0400000000
> > > [    0.377914] dw-pcie 4000000.pcie0: Failed to alloc and map MSI data
> > > [    0.378191] dw-pcie 4000000.pcie0: Failed to initialize host
> > > [    0.378255] dw-pcie: probe of 4000000.pcie0 failed with error -12
> > > 
> > > Mainline kernel 6.2-rc6 was used in that test.
> > > 
> > > The hardware supports 64-bit target addresses, so the patch "PCI: dwc:
> > > Add support for 64-bit MSI target address" should help with this
> > > particular failure.
> > > 
> > > 
> > > > 
> > > > Thanks,
> > > > Will
> > > > 
> > > > Will McVicker (2):
> > > >    PCI: dwc: Drop dependency on ZONE_DMA32
> > > > 
> > > > v6:
> > > >   * Retrying DMA allocation with 64-bit mask within the error
> > > > if-statement.
> > > >   * Use an int for the DMA mask instead of a bool and ternary operation.
> > > > 
> > > > v5:
> > > >   * Updated patch 2/2 to first try with a 32-bit DMA mask. On failure,
> > > >     retry with a 64-bit mask if supported.
> > > > 
> > > > v4:
> > > >   * Updated commit descriptions.
> > > >   * Renamed msi_64b -> msi_64bit.
> > > >   * Dropped msi_64bit ternary use.
> > > >   * Dropped export of dw_pcie_msi_capabilities.
> > > > 
> > > > v3:
> > > >    * Switched to a managed DMA allocation.
> > > >    * Simplified the DMA allocation cleanup.
> > > >    * Dropped msi_page from struct dw_pcie_rp.
> > > >    * Allocating a u64 instead of a full page.
> > > > 
> > > > v2:
> > > >    * Fixed build error caught by kernel test robot
> > > >    * Fixed error handling reported by Isaac Manjarres
> > > >   PCI: dwc: Add support for 64-bit MSI target address
> > > > 
> > > >   .../pci/controller/dwc/pcie-designware-host.c | 43 +++++++++----------
> > > >   drivers/pci/controller/dwc/pcie-designware.c  |  8 ++++
> > > >   drivers/pci/controller/dwc/pcie-designware.h  |  2 +-
> > > >   3 files changed, 30 insertions(+), 23 deletions(-)
> > > > 
> > > > 
> > > > base-commit: 568035b01cfb107af8d2e4bd2fb9aea22cf5b868
> > > 
> > > Thank you in advance.
> > > 
> > > Regards,
> > > Evgenii
> > > 
> > > 
> > > 
> > 
> 
> 
>