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

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Message-ID: <6b288f46-452d-6f92-728c-56c4100028cf@ti.com>
Date:   Fri, 6 Dec 2019 14:31:17 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>
CC:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andrew Murray <andrew.murray@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        <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 06/12/19 2:28 pm, Kunihiko Hayashi wrote:
> Hi Kishon,
> 
> On Fri, 6 Dec 2019 12:28:29 +0530 <kishon@...com> wrote:
> 
>> 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?
> 
> Surely it might be difficult for RC to decide the time to wait for EP.
> Since RC almost toggles PERST# in boot time, I'd like to think about
> how to prevent from first PERST# at least.

It can be prevented in the HW (If that's possible). I modify the cable 
connecting RC and EP to not propagate PERST#.

Thanks
Kishon