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Date:   Wed, 28 Sep 2022 17:52:26 +0000
From:   William McVicker <willmcvicker@google.com>
To:     Serge Semin <fancer.lancer@gmail.com>
Cc:     Jingoo Han <jingoohan1@gmail.com>,
        Gustavo Pimentel <gustavo.pimentel@synopsys.com>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Rob Herring <robh@kernel.org>,
        Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
        Bjorn Helgaas <bhelgaas@google.com>, kernel-team@android.com,
        Vidya Sagar <vidyas@nvidia.com>,
        Christoph Hellwig <hch@infradead.org>,
        Robin Murphy <robin.murphy@arm.com>, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target
 address
Message-ID: <YzSJ2ioEeRhHC6zn@google.com>
References: <20220825185026.3816331-1-willmcvicker@google.com>
 <20220825185026.3816331-3-willmcvicker@google.com>
 <20220928120510.rkdlnczytxuioxcn@mobilestation>
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On 09/28/2022, Serge Semin wrote:
> On Thu, Aug 25, 2022 at 06:50:25PM +0000, Will McVicker wrote:
> > Since not all devices require a 32-bit MSI address, add support to the
> > PCIe host driver to allow setting the DMA mask to 64-bits if the 32-bit
> > allocation fails. This allows kernels to disable ZONE_DMA32 and bounce
> > buffering (swiotlb) without risking not being able to get a 32-bit address
> > during DMA allocation.
> 
> What is a problem in having the ZONE_DMA32 enabled anyway?

On Android most devices don't have a 32-bit limitation. Several Android OEMs
have reported significant enough performance improvements after disabling
ZONE_DMA32. These include reducing memory usage, improving the time spent by
kswapd, improving direct reclaim, and improving app launch time.

So this patch series was introduced to remove the dependency on ZONE_DMA32 for
the DW PCIe drivers.

> 
> > 
> > Basically, in the slim chance that there are no 32-bit allocations
> > available, the current PCIe host driver will fail to allocate the msi_msg
> > page due to a DMA address overflow (seen in [1]). With this patch, the
> > PCIe host can retry the allocation with a 64-bit DMA mask if the current
> > PCIe device advertises 64-bit support via its MSI capabilities.
> > 
> > [1] https://lore.kernel.org/all/Yo0soniFborDl7+C@google.com/
> 
> Note the reported error isn't caused by the allocation procedure, but
> by the mapping procedure.
> 
> > 
> > Reported-by: kernel test robot <lkp@intel.com>
> > Signed-off-by: Will McVicker <willmcvicker@google.com>
> > Reviewed-by: Rob Herring <robh@kernel.org>
> > Acked-by: Jingoo Han <jingoohan1@gmail.com>
> > ---
> >  .../pci/controller/dwc/pcie-designware-host.c | 38 ++++++++++++++-----
> >  drivers/pci/controller/dwc/pcie-designware.c  |  8 ++++
> >  drivers/pci/controller/dwc/pcie-designware.h  |  1 +
> >  3 files changed, 38 insertions(+), 9 deletions(-)
> > 
> > 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.

Right. The DW PCIe device driver can set this flag during probe before calling
dw_pcie_host init() to ensure that we will always successfully allocate and map
the MSI target address (as required to return successfully from
dw_pcie_host_init()).

> 
> >  
> > -	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.

As mentioned above, we don't want to force this driver to require the kernel to
enable ZONE_DMA32. Since no I/O happens to this buffer, could we use
dma_alloc_attrs() with the DMA_ATTR_SKIP_CPU_SYNC and
DMA_ATTR_NO_KERNEL_MAPPING attribute? Would that address the "too expensive"
issues you're referring to?

With regards to the DMA mask, I'm okay with moving that out of the host
controller and into the DW PCIe device driver. That would address all of my
issues and we could just drop the logic for checking the PCI_MSI_FLAGS_64BIT.
However, I'm not the one you to convince to do that.

Regards,
Will

> 
> -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
> > 
> >