Re: [edk2-devel] [Patch V3 2/2] MdeModulePkg/Bus/Pci/PciBusDxe: Support PCIe Resizable BAR Capability

["Wu, Hao A" <hao.a.wu@intel.com>]

> -----Original Message-----
> From: Ni, Ray <ray.ni@intel.com>
> Sent: Tuesday, January 12, 2021 10:28 AM
> To: Laszlo Ersek <lersek@redhat.com>; devel@edk2.groups.io; Luo, Heng
> <heng.luo@intel.com>
> Cc: Wu, Hao A <hao.a.wu@intel.com>; Kinney, Michael D
> <michael.d.kinney@intel.com>
> Subject: RE: [edk2-devel] [Patch V3 2/2] MdeModulePkg/Bus/Pci/PciBusDxe:
> Support PCIe Resizable BAR Capability
> 
> > > It seems like the max BAR size is selected first, but if there's a
> > > "resource conflict" (running out of a particular resource type
> > > aperture), then the minimum BAR size is selected. I don't know what
> > > set of devices and/or resizable BARs this logic applies to, if there
> > > are multiple of them.
> 
> > > Per the PCIe specification (revision 5.0, version 0.9) 7.8.6:
> > >
> > >   Software determines, through a proprietary mechanism, what the
> > >   optimal size is for the resource, and programs that size via the BAR
> > >   Size field of the Resizable BAR Control register.
> > >
> > > Furthermore, Table 7-114 defines the Bar Size field of the control
> > > register stating:
> > >
> > >   The default value of this field is equal to the default size of the
> > >   address space that the BAR resource is requesting via the BAR's
> > >   read-only bits.
> > >
> > > Therefore the maximum size is not necessarily optimal, nor should
> > > the minimum size be considered the default.  In fact, [we] tested
> > > various handoff BAR sizes for [a particular] GPU and found that
> > > Windows didn't like the maximum BAR size.
> > >
> > > Elsewhere in the discussion [1] the AMD author of the kernel support
> > > for resizeable BARs indicates that FPGA devices might implement the
> > > REBAR capability as part of their standard PCI wrapper ([our]
> > > interpretation), but the BAR usage would be determined by the actual
> > > bitstream written to the device, therefore there might be a full
> > > bitmask for the BAR sizes supported by the device.
> > >
> > > [1]
> > > https://lists.freedesktop.org/archives/dri-devel/2021-January/thread
> > > .html
> > >
> > > It would certainly make sense for the firmware to take REBAR
> > > capabilities into account when sizing bridge apertures, but to
> > > generically enable extended BAR sizes would make lots of assumptions
> > > about the device usage and compatibility.
> > >
> > > [...] At least for GPUs the expectation would be a default, smaller
> > > compatibility size expanding to some representation that allows
> > > direct DMA to the entire memory of the card.
> >
> > So this patch should either be reverted; or minimally, the default
> > value of "PcdPcieResizableBarSupport" should be set to FALSE, as the
> > policy for BAR sizing doesn't look robust or portable.
> >
> >
> > General request for the future: if you implement some kind of policy
> > in core edk2, please at least *document* the policy somewhere. It's
> > unacceptable to have to decipher the source code for such a possibly
> > impactful change in the core. There is no need for a wiki page or an
> > RFC, but a sane bugzilla ticket and a sane commit message are required.
> >
> > (The documentation of the PCD in the "MdeModulePkg.dec" file is
> > unsatisfactory too, and the UNI file has not been updated at all.)
> >
> 
> 
> 
> Your understanding is correct. Original idea is to let platform supply the policy
> about what the optimal BAR size is for each resizable BAR.
> The current implementation is a try to avoid asking platform code for such
> policy because we thought it's a burden for platform to supply the policy data.
> 
> I agree that we set the PCD default value as disabled and after a period of
> study, we will understand whether a platform policy is really needed.


Hello Laszlo and Ray,

I saw Heng's patch series to
  1) Set the PCD default value to FALSE: https://edk2.groups.io/g/devel/message/70139
  2) Update the UNI file: https://edk2.groups.io/g/devel/message/70140
has got Reviewed-by/Acked-by tags from reviewers.

Do you have further comments for the series?
If not, I will merge this change in the next 24 hours.

Thanks in advance.

Best Regards,
Hao Wu


> 
> Thanks,
> Ray