linux-kernel - Re: [PATCH] PCI: aardvark: Don't touch PCIe registers if no card connected

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Message-ID: <20200630145848.GA3419143@bjorn-Precision-5520>
Date:   Tue, 30 Jun 2020 09:58:48 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Pali Rohár <pali@...nel.org>
Cc:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andrew Murray <amurray@...goodpenguin.co.uk>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Marek Behún <marek.behun@....cz>,
        Remi Pommarel <repk@...plefau.lt>,
        Tomasz Maciej Nowak <tmn505@...il.com>,
        Xogium <contact@...ium.me>, linux-pci@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: aardvark: Don't touch PCIe registers if no card
 connected

On Tue, Jun 30, 2020 at 04:04:20PM +0200, Pali Rohár wrote:
> On Tuesday 30 June 2020 08:51:27 Bjorn Helgaas wrote:
> > On Thu, May 28, 2020 at 06:38:09PM +0200, Pali Rohár wrote:
> > > On Thursday 28 May 2020 11:26:04 Bjorn Helgaas wrote:
> > > > On Thu, May 28, 2020 at 04:31:41PM +0200, Pali Rohár wrote:
> > > > > When there is no PCIe card connected and advk_pcie_rd_conf() or
> > > > > advk_pcie_wr_conf() is called for PCI bus which doesn't belong to emulated
> > > > > root bridge, the aardvark driver throws the following error message:
> > > > > 
> > > > >   advk-pcie d0070000.pcie: config read/write timed out
> > > > > 
> > > > > Obviously accessing PCIe registers of disconnected card is not possible.
> > > > > 
> > > > > Extend check in advk_pcie_valid_device() function for validating
> > > > > availability of PCIe bus. If PCIe link is down, then the device is marked
> > > > > as Not Found and the driver does not try to access these registers.
> > > > > 
> > > > > Signed-off-by: Pali Rohár <pali@...nel.org>
> > > > > ---
> > > > >  drivers/pci/controller/pci-aardvark.c | 3 +++
> > > > >  1 file changed, 3 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> > > > > index 90ff291c24f0..53a4cfd7d377 100644
> > > > > --- a/drivers/pci/controller/pci-aardvark.c
> > > > > +++ b/drivers/pci/controller/pci-aardvark.c
> > > > > @@ -644,6 +644,9 @@ static bool advk_pcie_valid_device(struct advk_pcie *pcie, struct pci_bus *bus,
> > > > >  	if ((bus->number == pcie->root_bus_nr) && PCI_SLOT(devfn) != 0)
> > > > >  		return false;
> > > > >  
> > > > > +	if (bus->number != pcie->root_bus_nr && !advk_pcie_link_up(pcie))
> > > > > +		return false;
> > > > 
> > > > I don't think this is the right fix.  This makes it racy because the
> > > > link may go down after we call advk_pcie_valid_device() but before we
> > > > perform the config read.
> > > 
> > > Yes, it is racy, but I do not think it cause problems. Trying to read
> > > PCIe registers when device is not connected cause just those timeouts,
> > > printing error message and increased delay in advk_pcie_wait_pio() due
> > > to polling loop. This patch reduce unnecessary access to PCIe registers
> > > when advk_pcie_wait_pio() polling just fail.
> > 
> > What happens when the device is removed after advk_pcie_link_up()
> > returns true, but before we actually do the config access?
> 
> Do you mean to remove device physically at runtime? I was told that our
> board would crash or issue reset. Removing device from mini PCIe slot
> without power off is not supported.

Right, I don't think PCIe mini cards support hotplug.

> Anyway, currently we are trying to read from device registers even when
> no device is connected. So when advk_pcie_link_up() returns true and
> after that device is not connected (somehow board and kernel would be
> still alive) I guess that it would behave as without applying this
> patch. So kernel starts reading from register and would wait until
> timeout expires. As device is not connected there would be no answer,
> so kernel print error message to dmesg (same as in commit message) and
> returns error that read failed.

OK, so if I understand correctly, checking advk_pcie_link_up() is
strictly an optimization.  If we guess wrong (e.g., after calling
advk_pcie_link_up(), the link went down because the card was removed,
DPC triggered, etc), the only bad thing is that we wait for a timeout;
it never causes a crash.

If that's the case, I'm fine with this.  But please add a comment to
that effect.

I think several other drivers check for the link being up because we
actually crash if we try to read config space when the link is down.
That's what I was trying to avoid here.

Bjorn