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

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Date:   Mon, 16 Jan 2017 11:34:01 +0100
From:   Lukasz Majewski <lukma@...x.de>
To:     Kishon Vijay Abraham I <kishon@...com>
Cc:     Joao Pinto <Joao.Pinto@...opsys.com>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        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

Dear All,

> + 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://git.kernel.org/cgit/linux/kernel/git/helgaas/pci.git/log/?h=pci/host-dra7xx
> >>>
> >>> 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://e2e.ti.com/support/arm/sitara_arm/f/791/t/566421
> > 
> > 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)

SPRUHZ6G – October 2014 – Revised June 2016 - page 6313. Maybe TRM is
not up to date?

> > 
> > 
> > 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 ;-)

:-) 

The details of my test cases and output are in the post [1].

> 
> I think these are 2 different approaches to keep the link at GEN1.
> Joao or Jingoo, do you have any suggestion here?

Please read through thread [1]

[1] https://e2e.ti.com/support/arm/sitara_arm/f/791/t/566421

> 
> > 
> >>
> >> 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.

Ah.... ok.
> 
> Thanks
> Kishon




Best regards,

Lukasz Majewski

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