linux-kernel - Re: [PATCH] pcie: ti: Provide patch to force GEN1 PCIe operation

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Message-ID: <24cff7e5-42d6-66e2-6f10-a6ea4ceb8899@synopsys.com>
Date:   Tue, 17 Jan 2017 10:19:53 +0000
From:   Joao Pinto <Joao.Pinto@...opsys.com>
To:     Kishon Vijay Abraham I <kishon@...com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Lukasz Majewski <lukma@...x.de>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>
CC:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        <linux-omap@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pcie: ti: Provide patch to force GEN1 PCIe operation


Hi,

Às 5:35 AM de 1/17/2017, Kishon Vijay Abraham I escreveu:
> Hi Joao,
> 
> On Monday 16 January 2017 10:31 PM, Joao Pinto wrote:
>>
>> Hi,
>>
>> Às 10:13 AM de 1/16/2017, Kishon Vijay Abraham I escreveu:
>>> + Joao, Jingoo
>>>
>>> Hi,
>>>
>>> On Monday 16 January 2017 03:01 PM, Lukasz Majewski wrote:
>>>> Hi Kishon,
>>>>
>>>>> Hi Łukasz,
>>>>>
>>>>> On Monday 16 January 2017 12:19 PM, Lukasz Majewski wrote:
>>>>>> Hi Kishon,
>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> On Sunday 15 January 2017 06:49 PM, Lukasz Majewski wrote:
>>>>>>>> Some devices (due to e.g. bad PCIe signal integrity) require to
>>>>>>>> run with forced GEN1 speed on PCIe bus.
>>>>>>>>
>>>>>>>> This patch changes the speed explicitly on dra7 based devices when
>>>>>>>> proper device tree attribute is defined for the PCIe controller.
>>>>>>>>
>>>>>>>> Signed-off-by: Lukasz Majewski <lukma@...x.de>
>>>>>>>
>>>>>>> Bjorn has already queued a patch to do the same thing
>>>>>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__git.kernel.org_cgit_linux_kernel_git_helgaas_pci.git_log_-3Fh-3Dpci_host-2Ddra7xx&d=DwIDaQ&c=DPL6_X_6JkXFx7AXWqB0tg&r=s2fO0hii0OGNOv9qQy_HRXy-xAJUD1NNoEcc3io_kx0&m=zD82T5n4WcL7Ga-NSY2NI7KE75xQ99hN-mW2yX46wQk&s=E8zk1CbKxGH-f3fw_WpXxFU-A8BLkgA8NusCaxk1SvA&e= 
>>>>>>
>>>>>> It seems like Bjorn only modifies CAP registers.
>>>>>
>>>>> The patch also modifies the LNKCTL2 register.
>>>>>>
>>>>>> He also needs to change register with 0x080C offset to actually
>>>>>> ( PCIECTRL_PL_WIDTH_SPEED_CTL )
>>>>>
>>>>> This bit is used to initiate speed change (after the link is
>>>>> initialized in GEN1). Resetting the bit (like what you have done
>>>>> here) prevents speed change.
>>>>
>>>> This is strange, but e2e advised me to do things as I did in the patch
>>>> to _force_ GEN1 operation on PCIe2 port [1] (AM5728)
>>>>
>>>> Link:
>>>> [1] https://urldefense.proofpoint.com/v2/url?u=https-3A__e2e.ti.com_support_arm_sitara-5Farm_f_791_t_566421&d=DwIDaQ&c=DPL6_X_6JkXFx7AXWqB0tg&r=s2fO0hii0OGNOv9qQy_HRXy-xAJUD1NNoEcc3io_kx0&m=zD82T5n4WcL7Ga-NSY2NI7KE75xQ99hN-mW2yX46wQk&s=uXLwglyRYqKpwp1JSxkOWmKpQ2wjfhgofpm8DCfquNw&e= 
>>>>
>>>> Both patches modify 0x5180 007C register to set GEN1 capability
>>>> (PCI_EXP_LNKCAP_SLS_2_5GB)
>>>>
>>>> The problem is with second register (in your patch):
>>>>
>>>> From SPRUHZ6G TRM:
>>>>
>>>> PCIECTRL_EP_DBICS_LNK_CAS_2 (0x5180 00A0)
>>>> - TRGT_LINK_SPEED (Reset 0x1) - "Target Link Speed" - no more
>>>>   description in TRM
>>>>
>>>> It is set to PCI_EXP_LNKCAP_SLS_2_5GB = 0x1, which is the same as
>>>> default /reset value.
>>>
>>> The default value is 0x2 (or else none of the cards would have enumerated in GEN2)
>>>>
>>>>
>>>> Could you clarify which way to _force_ PCIe GEN1 operation is correct?
>>>> Mine shows differences in lspci output (as posted in [1]).
>>>
>>> You'll see the difference even with the patch in Bjorn's tree ;-)
>>>
>>> I think these are 2 different approaches to keep the link at GEN1. Joao or
>>> Jingoo, do you have any suggestion here?
>>
>> I studied the Databook, and both approaches seem to be right, dependently of the
>> Core configuration and setup.
>>
>> The standard manual speed change sequence is:
>> a) Write to PCIE_CAP_TARGET_LINK_SPEED (indicating desired speed)
>> b) Clear "Directed Speed Change"
>> c) Set "Directed Speed Change"
>>
>> If "Directed Speed Change" is set (DEFAULT_GEN2_SPEED_CHANGE is the default
>> value), it will execute LTSSM to initiate speed change to Gen2 or Gen3, after
>> link is started in Gen1, and then the bit is automatically cleared.
>>
>> Lukasz is reseting this bit, in order to avoid the LTSSM to be executed, which
>> is correct. There is another way to prevent this automatic speed change, which
>> is to set GEN1 speed before link up which might be difficult in some setups, so
>> Kishon's also right.
> 
> Just for my understanding, why do you think this will be difficult in some setups?

It depends on the way some PCI Cores are configured.

>>
>> In my opinion Lukasz approach would be the one that might be more universal and
>> more "secure".
> 
> IMHO setting link control in the standard PCIe header space should be more
> universal. I'm not sure about the secure part though.

When I said secure, I misleaded you, sorry. I meant more robust.

> 
> Thanks
> Kishon
>>
>> Joao
>>
>>
>>>
>>>>
>>>>>
>>>>> IMO the better way is to set the LNKCTL2 to GEN1 instead of hacking
>>>>> the IP register.
>>>>
>>>> From the original patch description:
>>>>
>>>> "Add support to force Root Complex to work in GEN1 mode if so desired,
>>>> but don't force GEN1 mode on any board just yet."
>>>>
>>>> Are there any (floating around) patches allowing forcing GEN1 operation
>>>> on any board (I would like to reuse/port them to my current solution)?
>>>
>>> For setting to GEN1 mode, "max-link-speed" should be set to 1 in dt with the
>>> patch in Bjorn's tree.
>>>
>>> Thanks
>>> Kishon
>>>
>>