linux-kernel - Re: [PATCH v4 1/5] PCI: designware: add memory barrier after enabling region

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Date:	Fri, 11 Dec 2015 09:35:10 +0530
From:	Pratyush Anand <pratyush.anand@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Stanimir Varbanov <stanimir.varbanov@...aro.org>,
	Arnd Bergmann <arnd@...db.de>, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Bjorn Andersson <bjorn.andersson@...ymobile.com>
Subject: Re: [PATCH v4 1/5] PCI: designware: add memory barrier after enabling region

On Wed, Dec 9, 2015 at 3:53 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:

[...]

>> > >       dw_pcie_writel_rc(pp, PCIE_ATU_ENABLE, PCIE_ATU_CR2);
>> > > +     /*
>> > > +      * ensure that the ATU enable has been happaned before accessing
>> > > +      * pci configuration/io spaces through dw_pcie_cfg_[read|write].
>> > > +      */
>> > > +     wmb();
>> > >  }
>> > >
>>
>>
>> My understnading is that since writel() of dw_pcie_writel_rc() in
>> above code and readl(), writel() of dw_pcie_cfg_[read|write]() (which
>> will follow) goes through same device (ie PCIe host here). So, it is
>> guaranteed that 1st writel() will be executed before later
>> readl()/writel(). If that is true then we do not need any explicit
>> barrier here.
>>
>> Arnd, Russel: whats your opinion here.
>               ^l

Sorry :(

>
> writel() has a barrier _before_ the access but not after.
>
> The fact is that there's nothing which guarantees that the write will hit
> the hardware in a timely manner (forget any rules about PCI config space,
> the PCI ordering rules apply to the PCI bus, not to the ARM buses.)
>
> If you need this write to have hit the hardware before continuing, you
> need to read back from the same register.

OK, so better to replace wmb() with read back of control register.

>
> I'm just looking at this driver, trying to decipher what it's doing.  It
> _looks_ to me like it's reprogramming one of the outbound windows (IO?)
> so that configuration space can be accessed.  Doesn't this have the
> effect of disabling access to the IO segment of the PCI bus from the
> host CPU?
>
> What protections are there against other CPUs in the system issuing a
> PCI I/O read/write while this outbound window is programmed as
> configuration space?


Yes, that is an issue with this driver. Most of the host controller
has 4 or more viewpoints, and it is very easy to handle for them. But
there are few which has only two viewpoints. Do not know how to solve
it, so that it works for all.

~Pratyush
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