Re: [PATCH v2] PCI: dwc: Strengthen the MSI address allocation logic

[Ajay Agarwal <ajayagarwal@google.com>]

On Tue, Jan 16, 2024 at 09:02:48PM +0000, Robin Murphy wrote:
> On 2024-01-16 5:18 pm, Sajid Dalvi wrote:
> > On Tue, Jan 16, 2024 at 7:30 AM Robin Murphy <robin.murphy@arm.com> wrote:
> > > 
> > > On 2024-01-11 4:21 am, Ajay Agarwal wrote:
> > > > There can be platforms that do not use/have 32-bit DMA addresses
> > > > but want to enumerate endpoints which support only 32-bit MSI
> > > > address. The current implementation of 32-bit IOVA allocation can
> > > > fail for such platforms, eventually leading to the probe failure.
> > > > 
> > > > If there is a memory region reserved for the pci->dev, pick up
> > > > the MSI data from this region. This can be used by the platforms
> > > > described above.
> > > > 
> > > > Else, if the memory region is not reserved, try to allocate a
> > > > 32-bit IOVA. Additionally, if this allocation also fails, attempt
> > > > a 64-bit allocation for probe to be successful. If the 64-bit MSI
> > > > address is allocated, then the EPs supporting 32-bit MSI address
> > > > will not work.
> > > > 
> > > > Signed-off-by: Ajay Agarwal <ajayagarwal@google.com>
> > > > ---
> > > > Changelog since v1:
> > > >    - Use reserved memory, if it exists, to setup the MSI data
> > > >    - Fallback to 64-bit IOVA allocation if 32-bit allocation fails
> > > > 
> > > >    .../pci/controller/dwc/pcie-designware-host.c | 50 ++++++++++++++-----
> > > >    drivers/pci/controller/dwc/pcie-designware.h  |  1 +
> > > >    2 files changed, 39 insertions(+), 12 deletions(-)
> > > > 
> > > > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c
> > b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > index 7991f0e179b2..8c7c77b49ca8 100644
> > > > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > @@ -331,6 +331,8 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp
> > *pp)
> > > >        u64 *msi_vaddr;
> > > >        int ret;
> > > >        u32 ctrl, num_ctrls;
> > > > +     struct device_node *np;
> > > > +     struct resource r;
> > > > 
> > > >        for (ctrl = 0; ctrl < MAX_MSI_CTRLS; ctrl++)
> > > >                pp->irq_mask[ctrl] = ~0;
> > > > @@ -374,20 +376,44 @@ static int dw_pcie_msi_host_init(struct
> > dw_pcie_rp *pp)
> > > >         * order not to miss MSI TLPs from those devices the MSI target
> > > >         * address has to be within the lowest 4GB.
> > > >         *
> > > > -      * Note until there is a better alternative found the reservation
> > is
> > > > -      * done by allocating from the artificially limited DMA-coherent
> > > > -      * memory.
> > > > +      * Check if there is memory region reserved for this device. If
> > yes,
> > > > +      * pick up the msi_data from this region. This will be helpful for
> > > > +      * platforms that do not use/have 32-bit DMA addresses but want
> > to use
> > > > +      * endpoints which support only 32-bit MSI address.
> > > > +      * Else, if the memory region is not reserved, try to allocate a
> > 32-bit
> > > > +      * IOVA. Additionally, if this allocation also fails, attempt a
> > 64-bit
> > > > +      * allocation. If the 64-bit MSI address is allocated, then the
> > EPs
> > > > +      * supporting 32-bit MSI address will not work.
> > > >         */
> > > > -     ret = dma_set_coherent_mask(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");
> > > > +     np = of_parse_phandle(dev->of_node, "memory-region", 0);
> > > > +     if (np) {
> > > > +             ret = of_address_to_resource(np, 0, &r);
> > > 
> > > This is incorrect in several ways - reserved-memory nodes represent
> > > actual system memory, so can't be used to reserve arbitrary PCI memory
> > > space (which may be different if DMA offsets are involved); the whole
> > > purpose of going through the DMA API is to ensure we get a unique *bus*
> > > address. Obviously we don't want to reserve actual memory for something
> > > that functionally doesn't need it, but conversely having a
> > > reserved-memory region for an address which isn't memory would be
> > > nonsensical. And even if this *were* a viable approach, you haven't
> > > updated the DWC binding to allow it, nor defined a reserved-memory
> > > binding for the node itself.
> > > 
> > > If it was reasonable to put something in DT at all, then the logical
> > > thing would be a property expressing an MSI address directly on the
> > > controller node itself, but even that would be dictating software policy
> > > rather than describing an aspect of the platform itself. Plus this is
> > > far from the only driver with this concern, so it wouldn't make much
> > > sense to hack just one binding without all the others as well. The rest
> > > of the DT already describes everything an OS needs to know in order to
> > > decide an MSI address to use, it's just a matter of making this
> > > particular OS do a better job of putting it all together.
> > > 
> > > Thanks,
> > > Robin.
> > > 
> > 
> > Robin,
> > Needed some clarification.
> > It seems you are implying that the pcie device tree node should define a
> > property for the MSI address within the PCIe address space.
> > However, you also state that this would not be an ideal solution, and
> > would prefer using existing device tree constructs.
> > I am not sure what you mean by, " The rest of the DT already describes
> > everything."
> > Do you mean adding an "msi" reg to reg-names and defining the address
> > in the reg list?
> 
> No, I'm saying the closest this should come to DT at all is the possibility
> of the low-level driver hard-coding a platform-specific value for

I am assuming that you mean the platform driver (IOW, vendor driver) by
the "low-level" driver? Please confirm.

> pp->msi_data based on some platform-specific compatible, as Serge pointed to
> on v1.
>
Does this look ok to you? The expectation is that the pp->msi_data will
have to be populated by the platform driver if it wants to ensure the
support for all kinds of endpoints.

+       if (pp->msi_data)
+               return 0;
+
        ret = dma_set_coherent_mask(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_vaddr = dmam_alloc_coherent(dev, sizeof(u64), &pp->msi_data,
                                        GFP_KERNEL);
+       if (!msi_vaddr) {
+               dev_warn(dev, "Failed to alloc 32-bit MSI data. Attempting 64-bit now\n");
+               dma_set_coherent_mask(dev, DMA_BIT_MASK(64));
+               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);

> Otherwise, based on the system memory layout and dma-ranges of the
> controller node we have enough information to figure out what PCI bus
> address ranges can't collide with any valid DMA mapping of RAM, and thus
> generate a suitable MSI address, but that really wants to be a generic
> PCI-layer helper (which could also generically implement the various DMA API
> tricks as a fallback if necessary).
> 
> Thanks,
> Robin.