linux-kernel - Re: [RESEND PATCH v3 4/7] PCI: dwc: all: Modify dbi accessors to take dbi

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Date:   Mon, 13 Mar 2017 11:00:02 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Niklas Cassel <niklas.cassel@...s.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-omap@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
CC:     <nsekhar@...com>, Jingoo Han <jingoohan1@...il.com>,
        Richard Zhu <hongxing.zhu@....com>,
        Lucas Stach <l.stach@...gutronix.de>,
        Murali Karicheri <m-karicheri2@...com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Jesper Nilsson <jespern@...s.com>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        Gabriele Paoloni <gabriele.paoloni@...wei.com>
Subject: Re: [RESEND PATCH v3 4/7] PCI: dwc: all: Modify dbi accessors to take
 dbi_base as argument

Hi Niklas,

On Friday 10 March 2017 09:17 PM, Niklas Cassel wrote:
> 
> 
> On 03/10/2017 01:56 PM, Kishon Vijay Abraham I wrote:
>> Hi Niklas,
>>
>> On Friday 10 March 2017 06:01 PM, Niklas Cassel wrote:
>>> On 03/10/2017 12:36 PM, Kishon Vijay Abraham I wrote:
>>>> Hi,
>>>>
>>>> On Thursday 09 March 2017 08:35 PM, Niklas Cassel wrote:
>>>>> On 03/09/2017 03:48 PM, Niklas Cassel wrote:
>>>>>> On 03/09/2017 07:39 AM, Kishon Vijay Abraham I wrote:
>>>>>>> dwc has 2 dbi address space labeled dbics and dbics2. The existing
>>>>>>> helper to access dbi address space can access only dbics. However
>>>>>>> dbics2 has to be accessed for programming the BAR registers in the
>>>>>>> case of EP mode. This is in preparation for adding EP mode support
>>>>>>> to dwc driver.
>>>>>> Hello Kishon
>>>>>>
>>>>>> I don't really like the idea of adding an extra argument to every existing read/write.
>>>>>> Will not a read/write using dbi2 be quite uncommon compared to a read/write
>>>>>> using dbi?
>>>>>>
>>>>>> How about something like this:
>>>>>>
>>>>>> void __dw_pcie_writel(struct dw_pcie *pci, void __iomem *base, u32 reg, u32 val)
>>>>>> {
>>>>>>     if (pci->ops->writel_dbi)
>>>>>>         pci->ops->writel_dbi(pci, base, reg, val);
>>>>>>     else
>>>>>>         writel(val, base + reg);
>>>>>> }
>>>>>>
>>>>>> #define dw_pcie_writel_dbi(pci, reg, val) __dw_pcie_writel(pci, pci->dbi_base, reg, val)
>>>>>> #define dw_pcie_writel_dbi2(pci, reg, val) __dw_pcie_writel(pci, pci->dbi_base2, reg, val)
>>>>> Perhaps make dw_pcie_writel_dbi2 a function rather than a define,
>>>>> so we can return an error if pci->dbi_base2 == NULL.
>>>> Should we return an error? We don't return error for dbi_base either. I think
>>>> it should be sufficient to return errors while populating dbi_base or
>>>> dbi_base2. Otherwise it's a bug and should result in abort. Joao?
>>> Sorry for previous empty email.
>>>
>>>
>>> What I meant to write:
>>>
>>> Right now we do error checking for dbi_base in platform specific code
>>> and in pcie-designware-host.c:dw_pcie_host_init.
>> it's been done in dw_pcie_host_init not as an error checking but since it's
>> *optional* for certain platforms to populate dbi_base (i.e where dbi_base is
>> mapped to configuration space), host_init takes care of assigning dbi_base to
>> configuration space address.
> 
> What I'm afraid of is that we might get a NULL ptr dereference
> when using dw_pcie_writel_dbi2, if platform specific code has
> not populated dbi_base2.
> 
> Having a NULL check in generic code is just a fail safe if some
> platform specific code failed to NULL check.
> 
> The code in dw_pcie_host_init might have been written just
> to populate dbi_base when dbi is mapped to config space,
> but the end result is that if platform specific code did not
> populate dbi_base (and did not populate pp->cfg),
> we will return -ENOMEM.
> Which means that we can never get a NULL ptr dereference
> when using dw_pcie_writel_dbi.
> 
> It might be a good idea to have a NULL check in generic code,
> just as a fail safe, also for dw_pcie_ep_init.
> That way we know that we will not get a NULL ptr dereference
> when using dw_pcie_writel_dbi2.

All right, will add it then.

Thanks
Kishon