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

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  Re: [PATCH v4 1/5] PCI: designware: add memory barrier after enabling  region Stanimir Varbanov <stanimir.varbanov@linaro.org> - 2015-12-17 16:50 +0100
    Re: [PATCH v4 1/5] PCI: designware: add memory barrier after enabling region Pratyush Anand <pratyush.anand@gmail.com> - 2015-12-17 17:00 +0100

#1293987 — 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@arm.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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#1293996 — Re: [PATCH v4 1/5] PCI: designware: add memory barrier after enabling region

On Thu, Dec 17, 2015 at 9:15 PM, Stanimir Varbanov
<stanimir.varbanov@linaro.org> wrote:
>
> On 12/11/2015 06:05 AM, Pratyush Anand wrote:
> > On Wed, Dec 9, 2015 at 3:53 PM, Russell King - ARM Linux
> > <linux@arm.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?

For me it would be fine.

~Pratyush
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