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

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Date:   Tue, 30 Jun 2020 14:31:44 +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

Hello!

On Friday 29 May 2020 10:30:13 Pali Rohár wrote:
> On Thursday 28 May 2020 11:49:38 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.
> > > 
> > > 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.
> > 
> > Maybe advk_pcie_check_pio_status() and advk_pcie_wait_pio() could be
> > combined so we could get the correct error status as soon as it's
> > available, without waiting for a timeout?
> 
> Any idea how to achieve it?
> 
> First call is polling function advk_pcie_wait_pio() and second call is
> advk_pcie_check_pio_status() which just reads status register and prints
> error message to dmesg.
> 
> So for me it looks like that combining these two functions into one does
> not change anything. We always need to call polling code prior to
> checking status register. And therefore need to wait for timeout. Unless
> something like in this proposed patch is not used (to skip whole
> register access if it would fail).

So to answer your question, correct status is possible to retrieve only
after waiting for timeout. As status would be available only after
timeout expires.

Therefore my proposed patch in this (or some other) form is needed if we
want to prevent trying to read from registers and waiting for answer
when card is disconnected.

I would really like to see this issue fixed, so booting linux kernel on
board without connected PCIe card would not be delayed.

Thomas, Lorenzo, Bjorn: do you have any idea how to fix it differently?
Or if not, could be my proposed patch accepted in some form?

> > In any event, the "return PCIBIOS_SET_FAILED" needs to be fixed.  Most
> > callers of config read do not check for failure, but most of the ones
> > that do, check for "val == ~0".  Only a few check for a status of
> > other than PCIBIOS_SUCCESSFUL.
> > 
> > > > 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".
> 
> Now I see, "*val = 0xffffffff" should be really set when function
> advk_pcie_rd_conf() fails.

I have already sent separate patch which fixes this issue.

> > > > >  	return true;
> > > > >  }
> > > > >  
> > > > > -- 
> > > > > 2.20.1
> > > > >