linux-kernel - Re: [PATCH v2] fpga: dfl: afu: support Rev 2 of DFL Port feature

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Message-ID: <alpine.DEB.2.22.394.2401311610020.112016@sj-4150-psse-sw-opae-dev2>
Date: Wed, 31 Jan 2024 16:26:27 -0800 (PST)
From: matthew.gerlach@...ux.intel.com
To: Xu Yilun <yilun.xu@...ux.intel.com>
cc: hao.wu@...el.com, trix@...hat.com, mdf@...nel.org, yilun.xu@...el.com, 
    linux-fpga@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fpga: dfl: afu: support Rev 2 of DFL Port feature



On Wed, 31 Jan 2024, Xu Yilun wrote:

> On Tue, Jan 30, 2024 at 10:00:16AM -0800, matthew.gerlach@...ux.intel.com wrote:
>>
>>
>> On Tue, 30 Jan 2024, Xu Yilun wrote:
>>
>>> On Thu, Jan 25, 2024 at 03:37:15PM -0800, Matthew Gerlach wrote:
>>>> Revision 2 of the Device Feature List (DFL) Port feature
>>>> adds support for connecting the contents of the port to
>>>> multiple PCIe Physical Functions (PF).
>>>>
>>>> This new functionality requires changing the port reset
>>>> behavior during FPGA and software initialization from
>>>> revision 1 of the port feature. With revision 1, the initial
>>>> state of the logic inside the port was not guaranteed to
>>>> be valid until a port reset was performed by software during
>>>> driver initialization. With revision 2, the initial state
>>>> of the logic inside the port is guaranteed to be valid,
>>>> and a port reset is not required during driver initialization.
>>>>
>>>> This change in port reset behavior avoids a potential race
>>>> condition during PCI enumeration when a port is connected to
>>>> multiple PFs. Problems can occur if the driver attached to
>>>> the PF managing the port asserts reset in its probe function
>>>> when a driver attached to another PF accesses the port in its
>>>> own probe function. The potential problems include failed or hung
>>>
>>> Only racing during probe functions? I assume any time port_reset()
>>> would fail TLPs for the other PF. And port_reset() could be triggered
>>> at runtime by ioctl().
>>
>> Yes, a port_reset() triggered by ioctl could result in failed TLP for the
>> other PFs. The user space SW performing the ioctl needs to ensure all PFs
>> involved are properly quiesced before the port_reset is performed.
>
> How would user get an insight into other PF drivers to know everything
> is quiesced?  I mean do we need driver level management for this?

Since this is an FPGA, the number of other PFs and the drivers bound to 
those PFs depends on the FPGA image. There would also be user space 
software stacks involved with the other PFs as well. The user would have 
to ensure all the SW stacks and drivers are quiesced as appropriate for the 
FPGA image. I don't think the driver performing the port_reset() can know 
all the components to be able to provide any meaningful management.

Thanks,
Matthew

>
> Thanks,
> Yilun
>
>>
>> Do you want me to update the commit message with this information?
>>
>> Thanks,
>> Matthew
>
>