linux-kernel - Re: [PATCH 2/2] PCI: uniphier: Add checking whether PERST# is deasserted

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Message-ID: <c40da2f3-ea5d-b1fc-0190-f90f031eef4c@ti.com>
Date:   Fri, 6 Dec 2019 12:28:29 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andrew Murray <andrew.murray@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
CC:     <linux-pci@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Masami Hiramatsu <masami.hiramatsu@...aro.org>,
        Jassi Brar <jaswinder.singh@...aro.org>
Subject: Re: [PATCH 2/2] PCI: uniphier: Add checking whether PERST# is
 deasserted

Hi,

On 04/12/19 3:35 pm, Kunihiko Hayashi wrote:
> On Fri, 22 Nov 2019 20:53:16 +0900 <hayashi.kunihiko@...ionext.com> wrote:
> 
>> Hello Lorenzo,
>>
>> On Thu, 21 Nov 2019 16:47:05 +0000 <lorenzo.pieralisi@....com> wrote:
>>
>>> On Fri, Nov 08, 2019 at 04:30:27PM +0900, Kunihiko Hayashi wrote:
>>>>> However, If I understand correctly, doesn't your solution only work some
>>>>> of the time? What happens if you boot both machines at the same time,
>>>>> and PERST# isn't asserted prior to the kernel booting?
>>>>
>>>> I think it contains an annoying problem.
>>>>
>>>> If PERST# isn't toggled prior to the kernel booting, PERST# remains asserted
>>>> and the RC driver can't access PCI bus.
>>>>
>>>> As a result, this patch works and deasserts PERST# (and EP configuration will
>>>> be lost). So boot sequence needs to include deasserting PERST#.
>>>
>>> I am sorry but I have lost you. Can you explain to us why checking
>>> that PERST# is deasserted guarantees you that:
>>>
>>> - The EP has bootstrapped
>>> - It is safe not to toggle it again (and also skip
>>>    uniphier_pcie_ltssm_enable())
>>>
>>> Please provide details of the HW configuration so that we understand
>>> what's actually supposed to happen and why this patch fixes the
>>> issue you are facing.
>>
>> I tried to connect between the following boards, and do pci-epf-test:
>>   - "RC board": UniPhier ld20 board that has DWC RC controller
>>   - "EP board": UniPhier legacy board that has DWC EP controller
>>
>> This EP has power-on-state configuration, but it's necessary to set
>> class ID, BAR sizes, etc. after starting up.
>>
>> In case of that starting up RC board before EP board, the RC driver
>> can't establish link. So we need to boot EP board first.
> 
> At that point, I've considered why RC can't establish link,
> and found that the waitng time was too short.
> 
> - EP/RC: power on both boards
> 
> - RC: start up the kernel on RC board
> 
> - RC: wait for link up (long time enough)
> 
> - EP: start up the kernel on EP board
> 
> - EP: configurate pci-epf-test
> 
> When the endpoint  configuration is done and the EP driver enables LTSSM,
> the RC driver will quit from waiting for link up.
> 
> Currently DWC RC driver calls dwc_pcie_wait_for_link(), however,
> the function tries to link up 10 times only, that is defined
> as LINK_WAIT_MAX_RETRIES in pcie-designware.h, it's too short
> to configurate the endpoint.
> 
> Now the patch to bypass PERST# is not necessary.
> 
> Instead for DWC RC drivers, I think that the number of retries
> should be changed according to the usage.
> And the same issue remains with other RC drivers.

If EP is configured using Linux, then PERST# cannot be used as it's 
difficult to boot linux and initialize EP within the specified time 
interval. Can't you prevent PERST from being propagated at all?

Thanks
Kishon