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

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Message-ID: <5672D8A1.2080407@linaro.org>
Date:	Thu, 17 Dec 2015 17:45:37 +0200
From:	Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:	Pratyush Anand <pratyush.anand@...il.com>,
	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 12/11/2015 06:05 AM, Pratyush Anand wrote:
> 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.

Would the patch be acceptable if I replace wmb with read?

-- 
regards,
Stan
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