linux-kernel - Re: [PATCH v6 1/2] PCI: iproc: Retry request when CRS returned from EP

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Message-ID: <20170819202616.GA32411@bhelgaas-glaptop.roam.corp.google.com>
Date:   Sat, 19 Aug 2017 15:26:16 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Oza Oza <oza.oza@...adcom.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Jon Mason <jonmason@...adcom.com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        Andy Gospodarek <gospo@...adcom.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Oza Pawandeep <oza.pawandeep@...il.com>
Subject: Re: [PATCH v6 1/2] PCI: iproc: Retry request when CRS returned from
 EP

On Sun, Aug 20, 2017 at 01:02:09AM +0530, Oza Oza wrote:
> On Sat, Aug 19, 2017 at 11:56 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:

> > I think you should do something like this instead so you don't do the
> > MMIO read any more times than necessary:
> >
> >   static u32 iproc_pcie_cfg_retry(void __iomem *cfg_data_p)
> >   {
> >     u32 data;
> >     int timeout = CFG_RETRY_STATUS_TIMEOUT_US;
> >
> >     data = readl(cfg_data_p);
> >     while (data == CFG_RETRY_STATUS && timeout--) {
> >       udelay(1);
> >       data = readl(cfg_data_p);
> >     }
> >
> >     if (data == CFG_RETRY_STATUS)
> >       data = 0xffffffff;
> >     return data;
> >   }
> >
> >   static int iproc_pcie_config_read(...)
> >   {
> >     u32 data;
> >
> >     ...
> >     data = iproc_pcie_cfg_retry(cfg_data_p);
> >     if (size <= 2)
> >       *val = (data >> (8 * (where & 3))) & ((1 << (size * 8)) - 1);
> >
> > In the event of a timeout, this returns PCIBIOS_SUCCESSFUL and
> > 0xffffffff data.  That's what most other platforms do, and most
> > callers of the PCI config accessors check for that data instead of
> > checking the return code to see whether the access was successful.
> >
> I see one problem with this.
> we have Samsung NVMe which exposes 64-bit IO BAR.
> 0xffff0001.
> 
> and if you see __pci_read_base which does following in sequence.
> 
> pci_read_config_dword(dev, pos, &l);
> pci_write_config_dword(dev, pos, l | mask);
> pci_read_config_dword(dev, pos, &sz);
> pci_write_config_dword(dev, pos, l);
> 
> returning 0xffffffff would not be correct in that case.
> even if callers retry, they will end up in a loop if caller retries.
> 
> hence I was returning 0xffff0001, it is upto the upper layer to treat
> it as data or not.

In your patch, I don't think the upper layer will ever see 0xffff0001.
iproc_pcie_config_read() only updates *data if iproc_pcie_cfg_retry()
returns PCIBIOS_SUCCESSFUL.  And iproc_pcie_cfg_retry() only returns
PCIBIOS_SUCCESSFUL if it reads something other than 0xffff0001.

Even if you *did* return 0xffff0001 to upper layers, I think we'd have
a problem.  Here's an example scenario.  If we do a Function-Level
Reset, pcie_flr() starts the reset, then calls pci_flr_wait() to wait
until the device becomes ready again.  pci_flr_wait() thinks that any
PCI_COMMAND value other than 0xffffffff means the device is ready.

If the device returns CRS status and you return 0xffff0001 when
pci_flr_wait() reads PCI_COMMAND, it thinks that means the device is
ready, but it's not.

> let me know if this sounds like a problem to you as well.
> 
> so in my opinion returning 0xffffffff is not an option.
> 
> > For example, pci_flr_wait() assumes that if a read of PCI_COMMAND
> > returns ~0, it's because the device isn't ready yet, and we should
> > wait and retry.