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

["Ni, Ray" <ray.ni@intel.com>]

I've given R-b to the two patches. No comments from my side.

> -----Original Message-----
> From: Wu, Hao A <hao.a.wu@intel.com>
> Sent: Wednesday, January 13, 2021 2:00 PM
> To: Ni, Ray <ray.ni@intel.com>; Laszlo Ersek <lersek@redhat.com>;
> devel@edk2.groups.io; Luo, Heng <heng.luo@intel.com>
> Cc: 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
> 
> > -----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