Re: [PATCH V7 7/9] PCI/sysfs: Add a 10-Bit Tag sysfs file

[Dongdong Liu <liudongdong3@huawei.com>]

On 2021/8/10 1:37, Bjorn Helgaas wrote:
> On Sat, Aug 07, 2021 at 03:01:56PM +0800, Dongdong Liu wrote:
>>
>> On 2021/8/5 23:31, Bjorn Helgaas wrote:
>>> On Thu, Aug 05, 2021 at 04:37:39PM +0800, Dongdong Liu wrote:
>>>>
>>>>
>>>> On 2021/8/5 7:49, Bjorn Helgaas wrote:
>>>>> On Wed, Aug 04, 2021 at 09:47:06PM +0800, Dongdong Liu wrote:
>>>>>> PCIe spec 5.0 r1.0 section 2.2.6.2 says that if an Endpoint supports
>>>>>> sending Requests to other Endpoints (as opposed to host memory), the
>>>>>> Endpoint must not send 10-Bit Tag Requests to another given Endpoint
>>>>>> unless an implementation-specific mechanism determines that the Endpoint
>>>>>> supports 10-Bit Tag Completer capability. Add a 10bit_tag sysfs file,
>>>>>> write 0 to disable 10-Bit Tag Requester when the driver does not bind
>>>>>> the device if the peer device does not support the 10-Bit Tag Completer.
>>>>>> This will make P2P traffic safe. the 10bit_tag file content indicate
>>>>>> current 10-Bit Tag Requester Enable status.
>>>>>>
>>>>>> Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
>>>>>> ---
>>>>>>  Documentation/ABI/testing/sysfs-bus-pci | 16 +++++++-
>>>>>>  drivers/pci/pci-sysfs.c                 | 69 +++++++++++++++++++++++++++++++++
>>>>>>  2 files changed, 84 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
>>>>>> index 793cbb7..0e0c97d 100644
>>>>>> --- a/Documentation/ABI/testing/sysfs-bus-pci
>>>>>> +++ b/Documentation/ABI/testing/sysfs-bus-pci
>>>>>> @@ -139,7 +139,7 @@ Description:
>>>>>>  		binary file containing the Vital Product Data for the
>>>>>>  		device.  It should follow the VPD format defined in
>>>>>>  		PCI Specification 2.1 or 2.2, but users should consider
>>>>>> -		that some devices may have incorrectly formatted data.
>>>>>> +		that some devices may have incorrectly formatted data.
>>>>>>  		If the underlying VPD has a writable section then the
>>>>>>  		corresponding section of this file will be writable.
>>>>>>
>>>>>> @@ -407,3 +407,17 @@ Description:
>>>>>>
>>>>>>  		The file is writable if the PF is bound to a driver that
>>>>>>  		implements ->sriov_set_msix_vec_count().
>>>>>> +
>>>>>> +What:		/sys/bus/pci/devices/.../10bit_tag
>>>>>> +Date:		August 2021
>>>>>> +Contact:	Dongdong Liu <liudongdong3@huawei.com>
>>>>>> +Description:
>>>>>> +		If a PCI device support 10-Bit Tag Requester, will create the
>>>>>> +		10bit_tag sysfs file. The file is readable, the value
>>>>>> +		indicate current 10-Bit Tag Requester Enable.
>>>>>> +		1 - enabled, 0 - disabled.
>>>>>> +
>>>>>> +		The file is also writeable, the value only accept by write 0
>>>>>> +		to disable 10-Bit Tag Requester when the driver does not bind
>>>>>> +		the deivce. The typical use case is for p2pdma when the peer
>>>>>> +		device does not support 10-BIT Tag Completer.
>>>
>>>>>> +static ssize_t pci_10bit_tag_store(struct device *dev,
>>>>>> +				   struct device_attribute *attr,
>>>>>> +				   const char *buf, size_t count)
>>>>>> +{
>>>>>> +	struct pci_dev *pdev = to_pci_dev(dev);
>>>>>> +	bool enable;
>>>>>> +
>>>>>> +	if (kstrtobool(buf, &enable) < 0)
>>>>>> +		return -EINVAL;
>>>>>> +
>>>>>> +	if (enable != false )
>>>>>> +		return -EINVAL;
>>>>>
>>>>> Is this the same as "if (enable)"?
>>>> Yes, Will fix.
>>>
>>> I actually don't like the one-way nature of this.  When the hierarchy
>>> supports 10-bit tags, we automatically enable them during enumeration.
>>>
>>> Then we provide this sysfs file, but it can only *disable* 10-bit
>>> tags.  There's no way to re-enable them except by rebooting (or using
>>> setpci, I guess).
>>>
>>> Why can't we allow *enabling* them here if they're supported in this
>>> hierarchy?
>> Yes, we can also provide this sysfs to enable 10-bit tag for EP devices
>> when the hierarchy supports 10-bit tags.
>>
>> I do not want to provide sysfs to enable/disable 10-bit tag for RP
>> devices as I can not tell current if the the Function has outstanding
>> Non-Posted Requests, may need to unbind all the EP drivers under the
>> RP, and current seems no scenario need to do this. This will make things
>> more complex.
>
> You mean "no scenario requires disabling 10-bit tags and then
> re-enabling them"?
Just for Root Port devices.
> That may be true, but I'm still hesitant to
> provide a switch than can only be reversed by rebooting.
>
> This is similar to the issue Leon raised that it's not practical to
> reboot machines.  Maybe we accept a one-way switch if the sole purpose
> is to work around a hardware defect.  Or maybe a kernel parameter that
> disables 10-bit tags completely is the defect mitigation.  I think we
> probably need such a parameter in case a defect prevents us from
> booting far enough to use the sysfs switch.
Make sense, will provide sysfs to enable and disable 10-bit tag.

Thanks,
Dongdong
> .
>