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

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Message-ID: <68b85d0a-37d2-f83d-99ba-cd3936af4005@huawei.com>
Date:   Sat, 7 Aug 2021 15:01:56 +0800
From:   Dongdong Liu <liudongdong3@...wei.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     <hch@...radead.org>, <kw@...ux.com>, <logang@...tatee.com>,
        <leon@...nel.org>, <linux-pci@...r.kernel.org>,
        <rajur@...lsio.com>, <hverkuil-cisco@...all.nl>,
        <linux-media@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH V7 7/9] PCI/sysfs: Add a 10-Bit Tag sysfs file


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@...wei.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@...wei.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.

Thanks,
Dongdong
>
>>>> +	if (pdev->driver)
>>>> +		 return -EBUSY;
>>>> +
>>>> +	pcie_capability_clear_word(pdev, PCI_EXP_DEVCTL2,
>>>> +				   PCI_EXP_DEVCTL2_10BIT_TAG_REQ_EN);
>>>> +	pci_info(pdev, "disabled 10-Bit Tag Requester\n");
>>>> +
>>>> +	return count;
>>>> +}
> .
>