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

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Date:   Tue, 30 Jun 2020 08:51:27 -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 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?