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 15:49:51 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     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 Gustavo,

On 29/01/19 2:55 PM, Gustavo Pimentel wrote:
> 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.

Right, with with this fix cap_ptr is checked only once. This being a recursive
function, it makes sense to have the check only here instead of once in the
calling function and once here.
> 
>>  	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.

No, cap_id > PCI_CAP_ID_MAX is a base error case and it should checked before
returning the offset IMO.
> 
>>  	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?

so that we check cap_ptr only once.

Thanks
Kishon