netdev - Re: [PATCH net-next v2 0/2] sfc: Add EF100 BAR config support

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Date:   Thu, 14 Jul 2022 12:32:12 +0100
From:   Martin Habets <habetsm.xilinx@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, davem@...emloft.net,
        pabeni@...hat.com, edumazet@...gle.com, netdev@...r.kernel.org,
        ecree.xilinx@...il.com, linux-pci@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net-next v2 0/2] sfc: Add EF100 BAR config support

On Wed, Jul 13, 2022 at 11:48:04AM -0700, Jakub Kicinski wrote:
> On Wed, 13 Jul 2022 09:40:01 +0100 Martin Habets wrote:
> > > So it's switching between ethernet and vdpa? Isn't there a general
> > > problem for configuring vdpa capabilities (net vs storage etc) and
> > > shouldn't we seek to solve your BAR format switch in a similar fashion
> > > rather than adding PCI device attrs, which I believe is not done for
> > > anything vDPA-related?  
> > 
> > The initial support will be for vdpa net. vdpa block and RDMA will follow
> > later, and we also need to consider FPGA management.
> > 
> > When it comes to vDPA there is a "vdpa" tool that we intend to support.
> > This comes into play after we've switched a device into vdpa mode (using
> > this new file).
> > For a network device there is also "devlink" to consider. That could be used
> > to switch a device into vdpa mode, but it cannot be used to switch it
> > back (there is no netdev to operate on).
> > My current understanding is that we won't have this issue for RDMA.
> > For FPGA management there is no general configuration tool, just what
> > fpga_mgr exposes (drivers/fpga). We intend to remove the special PF
> > devices we have for this (PCI space is valuable), and use the normal
> > network device in stead. I can give more details on this if you want.
> > Worst case a special BAR config would be needed for this, but if needed I
> > expect we can restrict this to the NIC provisioning stage.
> > 
> > So there is a general problem I think. The solution here is something at
> > lower level, which is PCI in this case.
> > Another solution would be a proprietary tool, something we are off course
> > keen to avoid.
> 
> Okay. Indeed, we could easily bolt something onto devlink, I'd think
> but I don't know the space enough to push for one solution over
> another. 
> 
> Please try to document the problem and the solution... somewhere, tho.
> Otherwise the chances that the next vendor with this problem follows
> the same approach fall from low to none.

Yeah, good point. The obvious thing would be to create a
 Documentation/networking/device_drivers/ethernet/sfc/sfc/rst
Is that generic enough for other vendors to find out, or there a better place?
I can do a follow-up patch for this.

Martin