Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target address

[Lorenzo Pieralisi <lpieralisi@kernel.org>]

On Wed, Sep 28, 2022 at 03:05:10PM +0300, Serge Semin wrote:
> On Thu, Aug 25, 2022 at 06:50:25PM +0000, Will McVicker wrote:

[...]

> > 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.

The MSI cap shows that - AFAICS - the RP can be programmed with
a 64-bit MSI(DMA) address. It is fair to say that this is not
enough to guarantee that the DMA mask for the host bridge can be
inferred to be 64-bit though.

I am inclined to drop this patch (only) from the PCI queue because
it is unclear to me whether it really does the right thing.

Lorenzo

> > -	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
> > 
> >