Re: [PATCH v2] PCI: vmd: Clear PCI_INTERRUPT_LINE for VMD sub-devices

[Nirmal Patel <nirmal.patel@linux.intel.com>]

On Fri, 13 Sep 2024 16:25:41 +0530
Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org> wrote:

> On Thu, Sep 12, 2024 at 05:01:57PM -0700, Dan Williams wrote:
> > Nirmal Patel wrote:  
> > > On Thu, 12 Sep 2024 11:10:21 -0700
> > > Dan Williams <dan.j.williams@intel.com> wrote:
> > >   
> > > > Manivannan Sadhasivam wrote:
> > > > [..]  
> > > > > I don't think the issue should be constrained to VMD only.
> > > > > Based on my conversation with Nirmal [1], I understood that
> > > > > it is SPDK that makes wrong assumption if the device's
> > > > > PCI_INTERRUPT_LINE is non-zero (and I assumed that other
> > > > > application could do the same).    
> > > > 
> > > > I am skeptical one can find an example of an application that
> > > > gets similarly confused. SPDK is not a typical consumer of PCI
> > > > device information.
> > > >   
> > > > > In that case, how it can be classified as the "idiosyncracy"
> > > > > of VMD?    
> > > > 
> > > > If VMD were a typical PCIe switch, firmware would have already
> > > > fixed up these values. In fact this problem could likely also
> > > > be fixed in platform firmware, but the history of VMD is to
> > > > leak workaround after workaround into the kernel.  
> > > 
> > > This is not VMD leaking workaround in kernel, rather the patch is
> > > trying to keep fix limited to VMD driver.  
> > 
> > Oh, ok, I see that now, however...
> >   
> > > I tried over 10 different NVMes and only 1 vendor has
> > > PCI_INTERRUPT_LINE register set to 0xFF.  The platform firmware
> > > doesn't change that with or without VMD.  
> > 
> > ...SPDK is still asserting that it wants to be the NVME host driver
> > in userspace. As part of that it gets to keep all the pieces and
> > must realize that a device that has MSI/-X enabled is not using INTx
> > regardless of that register value.
> > 
> > Do not force the kernel to abide by SPDK expectations when the PCI
> > core / Linux-NVME driver contract does not need the
> > PCI_INTERRUPT_LINE cleared. If SPDK is taking over NVME, it gets to
> > take over *all* of it.  
> 
> In that case, we do not need a fix at all (even for VMD). My initial
> assumption was that, some other userspace applications may also
> behave the same way as SPDK. But I agree with you that unless we hear
> about them, it doesn't warrant a fix in the kernel. Thanks!

Okay, We can drop this patch for now. Thanks.
> 
> - Mani
>