linux-kernel - Re: [RFC PATCH 05/14] PCI: add access functions for PCIe capabilities to hide PCIe spec differences

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Date:	Thu, 12 Jul 2012 10:56:44 +0800
From:	Jiang Liu <jiang.liu@...wei.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	Jiang Liu <liuj97@...il.com>, Don Dutile <ddutile@...hat.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Taku Izumi <izumi.taku@...fujitsu.com>,
	"Rafael J . Wysocki" <rjw@...k.pl>,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	Yijing Wang <wangyijing@...wei.com>,
	Keping Chen <chenkeping@...wei.com>,
	<linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>
Subject: Re: [RFC PATCH 05/14] PCI: add access functions for PCIe capabilities
 to hide PCIe spec differences

On 2012-7-12 1:52, Bjorn Helgaas wrote:
>> Hi Bjorn,
>>         Seems it would be better to return error code for unimplemented
>> registers, otherwise following code will becomes more complex. A special
>> error code for unimplemented registers, such as -EIO?
> 
> I think you're asking about returning error for *reads* of
> unimplemented registers?  I guess I still think it's OK to completely
> hide the v1 nastiness inside these accessors, and return success with
> a zero value when reading.  Having several different error returns
> seems like overkill for this case.  Nobody wants to distinguish
> between different reasons for failure.
> 
> I'm actually not sure that it's worth returning an error even when
> *writing* an unimplemented register.  What if we return success and
> just drop the write?
> 
> Maybe these should even be void functions.  It feels like the only
> real use of the return value is to detect programmer error, and I
> don't think that's very effective.  If we remove the return values,
> people will have to focus on the *data*, which seems more important
> anyway.
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.

>> static void rtl_disable_clock_request(struct pci_dev *pdev)
>> {
>>         u16 ctl;
>>
>>         if (!pci_pcie_capability_read_word(pdev, PCI_EXP_LNKCTL, &ctl)) {
>>                 ctl &= ~PCI_EXP_LNKCTL_CLKREQ_EN;
>>                 pci_pcie_capability_write_word(pdev, PCI_EXP_LNKCTL, ctl);
>>         }
>> }
> 
> I would write that as:
> 
>     if (!pci_is_pcie(pdev)
>         return;
> 
>     pci_pcie_capability_read_word(pdev, PCI_EXP_LNKCTL, &ctl);
>     if (ctl & PCI_EXP_LNKCTL_CLKREQ_EN)
>         pci_pcie_capability_write_word(pdev, PCI_EXP_LNKCTL, ctl &
> ~PCI_EXP_LNKCTL_CLKREQ_EN);
> 
> which does the right thing regardless of what we do for return values,
> and saves a config write in the case where LNKCTL is implemented and
> CLKREQ_EN is already cleared.
When clearing a flag, we could do that. But if we are trying to set a
flag, it would be better to make sure the target register does exist.

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