linux-kernel - Re: [PATCH v9 2/4] PCI: Disable PCIe Relaxed Ordering if unsupported

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Date:   Wed, 9 Aug 2017 21:42:48 +0800
From:   Ding Tianhong <dingtianhong@...wei.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     <mark.rutland@....com>, <gabriele.paoloni@...wei.com>,
        <asit.k.mallick@...el.com>, <catalin.marinas@....com>,
        <will.deacon@....com>, <linuxarm@...wei.com>,
        <alexander.duyck@...il.com>, <ashok.raj@...el.com>,
        <jeffrey.t.kirsher@...el.com>, <linux-pci@...r.kernel.org>,
        <ganeshgr@...lsio.com>, <Bob.Shaw@....com>, <leedom@...lsio.com>,
        <patrick.j.cramer@...el.com>, <bhelgaas@...gle.com>,
        <werner@...lsio.com>, <linux-arm-kernel@...ts.infradead.org>,
        <amira@...lanox.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <David.Laight@...lab.com>,
        <Suravee.Suthikulpanit@....com>, <robin.murphy@....com>,
        <davem@...emloft.net>, <l.stach@...gutronix.de>
Subject: Re: [PATCH v9 2/4] PCI: Disable PCIe Relaxed Ordering if unsupported

On 2017/8/9 11:25, Bjorn Helgaas wrote:
> On Tue, Aug 08, 2017 at 09:22:39PM -0500, Bjorn Helgaas wrote:
>> On Sat, Aug 05, 2017 at 03:15:11PM +0800, Ding Tianhong wrote:
>>> When bit4 is set in the PCIe Device Control register, it indicates

> After looking at the driver, I wonder if it would be simpler like
> this:
> 
>   int pcie_relaxed_ordering_enabled(struct pci_dev *dev)
>   {
>     u16 ctl;
> 
>     pcie_capability_read_word(dev, PCI_EXP_DEVCTL, &ctl);
>     return ctl & PCI_EXP_DEVCTL_RELAX_EN;
>   }
>   EXPORT_SYMBOL(pcie_relaxed_ordering_enabled);
> 
>   static void pci_configure_relaxed_ordering(struct pci_dev *dev)
>   {
>     struct pci_dev *root;
> 
>     if (dev->is_virtfn)
>       return;  /* PCI_EXP_DEVCTL_RELAX_EN is RsvdP in VFs */
> 
>     if (!pcie_relaxed_ordering_enabled(dev))
>       return;
> 
>     /*
>      * For now, we only deal with Relaxed Ordering issues with Root
>      * Ports.  Peer-to-peer DMA is another can of worms.
>      */
>     root = pci_find_pcie_root_port(dev);
>     if (!root)
>       return;
> 
>     if (root->relaxed_ordering_broken)
>       pcie_capability_clear_word(dev, PCI_EXP_DEVCTL,
>                                  PCI_EXP_DEVCTL_RELAX_EN);
>   }
> 
> This doesn't check every intervening switch, but I don't think we know
> about any issues except with root ports.
> 

Yes

> And the driver could do:
> 
>   if (!pcie_relaxed_ordering_enabled(pdev))
>     adapter->flags |= ROOT_NO_RELAXED_ORDERING;
> 
> The driver code wouldn't show anything about coherent memory vs.
> peer-to-peer, but we really don't have a clue about how to handle that
> yet anyway.
> 
> I guess this is back to exactly what you proposed, except that I
> changed the name of pcie_relaxed_ordering_supported() to
> pcie_relaxed_ordering_enabled(), which I think is slightly more
> specific from the device's point of view.
> 

OK, looks like we reach a consensus finally, I will follow your new opinion and resend, thanks.

Ding

> Bjorn
> 
> .
>