linux-kernel - Re: [v3] PCI: Avoid unsync of LTR mechanism configuration

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Date:   Wed, 3 Feb 2021 10:05:18 +0200
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Mingchuang Qiao <mingchuang.qiao@...iatek.com>
Cc:     bhelgaas@...gle.com, matthias.bgg@...il.com,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, haijun.liu@...iatek.com,
        lambert.wang@...iatek.com, kerun.zhu@...iatek.com,
        alex.williamson@...hat.com, rjw@...ysocki.net,
        utkarsh.h.patel@...el.com
Subject: Re: [v3] PCI: Avoid unsync of LTR mechanism configuration

On Wed, Feb 03, 2021 at 10:14:01AM +0800, Mingchuang Qiao wrote:
> Hi,
> 
> On Mon, 2021-02-01 at 13:32 +0200, Mika Westerberg wrote:
> > Hi,
> > 
> > On Fri, Jan 29, 2021 at 03:11:37PM +0800, mingchuang.qiao@...iatek.com wrote:
> > > From: Mingchuang Qiao <mingchuang.qiao@...iatek.com>
> > > 
> > > In bus scan flow, the "LTR Mechanism Enable" bit of DEVCTL2 register is
> > > configured in pci_configure_ltr(). If device and bridge both support LTR
> > > mechanism, the "LTR Mechanism Enable" bit of device and bridge will be
> > > enabled in DEVCTL2 register. And pci_dev->ltr_path will be set as 1.
> > > 
> > > If PCIe link goes down when device resets, the "LTR Mechanism Enable" bit
> > > of bridge will change to 0 according to PCIe r5.0, sec 7.5.3.16. However,
> > > the pci_dev->ltr_path value of bridge is still 1.
> > > 
> > > For following conditions, check and re-configure "LTR Mechanism Enable" bit
> > > of bridge to make "LTR Mechanism Enable" bit mtach ltr_path value.
> > 
> > Typo mtach -> match.
> > 
> > >    -before configuring device's LTR for hot-remove/hot-add
> > >    -before restoring device's DEVCTL2 register when restore device state
> > > 
> > > Signed-off-by: Mingchuang Qiao <mingchuang.qiao@...iatek.com>
> > > ---
> > > changes of v2
> > >  -modify patch description
> > >  -reconfigure bridge's LTR before restoring device DEVCTL2 register
> > > changes of v3
> > >  -call pci_reconfigure_bridge_ltr() in probe.c
> > 
> > Hmm, which part of this patch takes care of the reset path? It is not
> > entirely clear to me at least.
> > 
> 
> When device resets and link goes down, there seems to have two methods
> to recover for software. 
>    -One is that trigger device removal and rescan.
>    -The other is that restore device with pci_restore_state() after link
> comes back up.
> For above both scenarios, we need check and reconfigure "LTR Mechanism
> Enable" bit of bridge. It's also this patch intends to do.
>    -For the rescan scenario, it's done in pci_configure_ltr().
>    -For the restore scenario, it's done in pci_restore_pcie_state().

Okay, thanks for the clarification!