linux-kernel - Re: [PATCH v3] PCI: aardvark: Don't touch PCIe registers if no card connected

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Message-ID: <20200709113509.GB19638@e121166-lin.cambridge.arm.com>
Date:   Thu, 9 Jul 2020 12:35:09 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Pali Rohár <pali@...nel.org>
Cc:     Thomas Petazzoni <thomas.petazzoni@...tlin.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 v3] PCI: aardvark: Don't touch PCIe registers if no card
 connected

On Thu, Jul 02, 2020 at 10:30:36AM +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.
> 
> This is just an optimization to prevent accessing PCIe registers when card
> is disconnected. Trying to access PCIe registers of disconnected card does
> not cause any crash, kernel just needs to wait for a timeout. So if card
> disappear immediately after checking for PCIe link (before accessing PCIe
> registers), it does not cause any problems.
> 
> Signed-off-by: Pali Rohár <pali@...nel.org>
> 
> ---
> Changes in V3:
> * Add comment to the code
> Changes in V2:
> * Update commit message, mention that this is optimization
> ---
>  drivers/pci/controller/pci-aardvark.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> index 90ff291c24f0..d18f389b36a1 100644
> --- a/drivers/pci/controller/pci-aardvark.c
> +++ b/drivers/pci/controller/pci-aardvark.c
> @@ -644,6 +644,13 @@ 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 the link goes down after we check for link-up, nothing bad
> +	 * happens but the config access times out.
> +	 */
> +	if (bus->number != pcie->root_bus_nr && !advk_pcie_link_up(pcie))
> +		return false;
> +
>  	return true;
>  }

Question: this basically means that you can only effectively enumerate
bus number == root_bus_nr and AFAICS if at probe the link did not
come up it will never do, will it ?

Isn't this equivalent to limiting the bus numbers the bridge is capable
of handling ?

Reworded: if in advk_pcie_setup_hw() the link does not come up, what's
the point of trying to enumerate the bus hierarchy below the root bus ?

Thanks,
Lorenzo