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

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Date:   Thu, 28 May 2020 18:38:09 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Bjorn Helgaas <helgaas@...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 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.

I think it is a good idea to not call blocking advk_pcie_wait_pio() when
it is not needed. We could have faster enumeration of PCIe buses when
card is not connected.

> I have no objection to removing the "config read/write timed out"
> message.  The "return PCIBIOS_SET_FAILED" in the read case probably
> should be augmented by setting "*val = 0xffffffff".
> 
> >  	return true;
> >  }
> >  
> > -- 
> > 2.20.1
> >