Re: [RFC PATCH 05/14] PCI: add access functions for PCIe capabilitiesto hide PCIe spec differences

From: Bjorn Helgaas
Date: Mon Jul 16 2012 - 20:14:41 EST

On Mon, Jul 16, 2012 at 6:09 PM, Jiang Liu <liuj97@xxxxxxxxx> wrote:
> On 07/17/2012 01:29 AM, Bjorn Helgaas wrote:
>> On Sun, Jul 15, 2012 at 10:47 AM, Jiang Liu <liuj97@xxxxxxxxx> wrote:
>>> On 07/13/2012 04:49 AM, Bjorn Helgaas wrote:
>>>>> Hi Bjorn,
>>>>> It's a little risk to change these PCIe capabilities access
>>>>> functions as void. On some platform with hardware error detecting/correcting
>>>>> capabilities, such as EEH on Power, it would be better to return
>>>>> error code if hardware error happens during accessing configuration registers.
>>>>> As I know, coming Intel Xeon processor may provide PCIe hardware
>>>>> error detecting capability similar to EEH on power.
>>>> I guess I'm playing devil's advocate here. As a general rule, people
>>>> don't check the return value of pci_read_config_*() or
>>>> pci_write_config_*(). Unless you change them all, most callers of
>>>> pci_pcie_capability_read_*() and _write_*() won't check the returns
>>>> either. So I'm not sure return values are an effective way to detect
>>>> those hardware errors.
>>>> How do these EEH errors get detected or reported today? Do the
>>>> drivers check every config access for success? Adding those checks
>>>> and figuring out how to handle errors at every possible point doesn't
>>>> seem like a recipe for success.
>>> Hi Bjorn,
>>> Sorry for later reply, on travel these days.
>>> Yeah, it's true that most driver doesn't check return values of configuration
>>> access functions, but there are still some drivers which do check return value of
>>> pci_read_config_xxx(). For example, pciehp driver checks return value of CFG access
>>> functions.
>>> It's not realistic to enhance all drivers, but we may focus on a small set of
>>> drivers for hardwares on specific high-end servers. For RAS features, we can never provide
>>> perfect solutions, so we prefer some improvements. After all a small improvement is still
>>> an improvement:)
>>> I'm only familiar with PCI on IA64 and x86. For PowerPC, I just know that the OS
>>> may query firmware whether there's some hardware faults if pci_cfg_read_xxx() returns
>>> all 1s. For PCI on IA64, SAL may handle PCI hardware errors and return error code to
>>> pci_cfg_read_xxx(). For x86, I think it will have some mechanisms to report hardware faults
>>> like SAL on IA64.
>>> So how about keeping consistence with pci_cfg_read_xxx() and pci_user_cfg_read_xxx()?
>> My goal is "the caller should never have to know whether this is a v1
>> or v2 capability." Returning any error other than one passed along
>> from pci_read/write_config_xxx() means we miss that goal. Perhaps the
>> goal is unattainable, but I haven't been convinced yet.
>> I think hardware error detection is irrelevant to this discussion.
>> After reading Documentation/PCI/pci-error-recovery.txt, I'm even less
>> convinced that checking return values from pci_read/write_config_xxx()
>> or pci_pcie_capability_read/write_xxx() is a useful way to detect
>> hardware errors.
>> Having drivers detect hardware failures by checking for config access
>> errors is neither necessary nor sufficient. It's not necessary
>> because a platform can implement a config accessor that checks *every*
>> access and reports failures to the driver via the pci_error_handler
>> framework. It's not sufficient because config accesses are rare
>> (usually only at init-time), and hardware failures may happen at
>> arbitrary other times.
>> In my opinion, the only relevant question is whether a caller of
>> pci_pcie_capability_read/write_xxx() needs to know whether a register
>> is implemented (i.e., we have a v2 capability) or not. For reads, I
>> don't think there's a case where fabricating a value of zero when
>> reading an unimplemented register is a problem.
>> Writes are obviously more interesting, but I'm still not sure there's
>> a case where silently dropping a write to an unimplemented register is
>> a problem. The "capability" registers are read-only, so there's no
>> problem if we drop writes to them. The "status" registers are
>> generally RO or RW1C, where it's only meaningful to write a non-zero
>> value if you're previously *read* a non-zero value. The "control"
>> registers are often RW, of course, but generally it's only meaningful
>> to write a non-zero value when a non-zero bit in the "capability"
>> register has previously told you that something is supported.
> Hi Bjorn,
> I'm convinced by you that we shouldn't return error when accessing
> an unimplemented PCIe capabilities register and just hide the differences
> among V1/V2 specification. Then how about returning error from
> "pci_read/write_config_xxx()" to callers of pci_pcie_capabilitiy_read/write_xxx()?
> I still prefer to return error code to keep consistence with other configuration
> space access interfaces:)

I think it's fine to return the status of pci_read/write_config_xxx(), e.g.,

int pci_pcie_cap_read_word(...)
if (<implemented>)
return pci_read_config_word(...);

return 0;

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