linux-kernel - Re: [PATCH 18/24] PCI: dwc: Fix dw_pcie_ep_find_capability to return correct capability offset

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Date:   Tue, 29 Jan 2019 09:25:09 +0000
From:   Gustavo Pimentel <gustavo.pimentel@...opsys.com>
To:     Kishon Vijay Abraham I <kishon@...com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Rob Herring <robh+dt@...nel.org>,
        "Lorenzo Pieralisi" <lorenzo.pieralisi@....com>
CC:     Jingoo Han <jingoohan1@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Murali Karicheri <m-karicheri2@...com>,
        Jesper Nilsson <jesper.nilsson@...s.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arm-kernel@...s.com" <linux-arm-kernel@...s.com>
Subject: Re: [PATCH 18/24] PCI: dwc: Fix dw_pcie_ep_find_capability to return
 correct capability offset

Hi Kishon,

On 14/01/2019 13:24, Kishon Vijay Abraham I wrote:
> commit beb4641a787df79a ("PCI: dwc: Add MSI-X callbacks handler") while
> adding MSI-X callback handler, introduced dw_pcie_ep_find_capability and
> __dw_pcie_ep_find_next_cap for finding the MSI and MSIX capability.
> 
> However if MSI or MSIX capability is the last capability (i.e there are
> no additional items in the capabilities list and the Next Capability
> Pointer is set to '0'), __dw_pcie_ep_find_next_cap will return '0'
> even though MSI or MSIX capability may be present. This is because of
> incorrect ordering of "next_cap_ptr" check. Fix it here.
> 
> Fixes: beb4641a787df79a142 ("PCI: dwc: Add MSI-X callbacks handler")
> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
> ---
>  drivers/pci/controller/dwc/pcie-designware-ep.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-designware-ep.c b/drivers/pci/controller/dwc/pcie-designware-ep.c
> index d5144781005b..cd51b008858c 100644
> --- a/drivers/pci/controller/dwc/pcie-designware-ep.c
> +++ b/drivers/pci/controller/dwc/pcie-designware-ep.c
> @@ -46,16 +46,19 @@ static u8 __dw_pcie_ep_find_next_cap(struct dw_pcie *pci, u8 cap_ptr,
>  	u8 cap_id, next_cap_ptr;
>  	u16 reg;
>  
> +	if (!cap_ptr)
> +		return 0;
> +

Supposedly this was already verified by the function that calls this one.

>  	reg = dw_pcie_readw_dbi(pci, cap_ptr);
> -	next_cap_ptr = (reg & 0xff00) >> 8;
>  	cap_id = (reg & 0x00ff);
>  
> -	if (!next_cap_ptr || cap_id > PCI_CAP_ID_MAX)
> +	if (cap_id > PCI_CAP_ID_MAX)
>  		return 0;
>  
>  	if (cap_id == cap)
>  		return cap_ptr;
>  
> +	next_cap_ptr = (reg & 0xff00) >> 8;

This fix seems to be a bit overdone, especially when you only need to swap the
if blocks order to achieve the desired goal.

>  	return __dw_pcie_ep_find_next_cap(pci, next_cap_ptr, cap);
>  }
>  
> @@ -67,9 +70,6 @@ static u8 dw_pcie_ep_find_capability(struct dw_pcie *pci, u8 cap)
>  	reg = dw_pcie_readw_dbi(pci, PCI_CAPABILITY_LIST);
>  	next_cap_ptr = (reg & 0x00ff);
>  
> -	if (!next_cap_ptr)
> -		return 0;
> -

Why remove it?
If pointer is null, why to jump to another function to check is the the same
pointer is null?

>  	return __dw_pcie_ep_find_next_cap(pci, next_cap_ptr, cap);
>  }
>  
>