> -----Original Message----- > From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Laszlo > Ersek > Sent: Friday, November 3, 2023 12:19 AM > To: devel@edk2.groups.io; jlo...@gmail.com > Cc: Leif Lindholm <quic_llind...@quicinc.com>; Ard Biesheuvel > <ardb+tianoc...@kernel.org>; Sami Mujawar <sami.muja...@arm.com>; > Wang, Jian J <jian.j.w...@intel.com>; Gao, Liming > <gaolim...@byosoft.com.cn>; Wu, Hao A <hao.a...@intel.com>; Ni, Ray > <ray...@intel.com>; Pedro Falcato <pedro.falc...@gmail.com>; Michael > Brown <mc...@ipxe.org> > Subject: Re: [edk2-devel] [PATCH] MdeModulePkg/PciHostBridgeDxe: Add > readback after final Cfg-Write > > On 11/3/23 01:03, Joe L wrote: > > From: joelopez333 <jlo...@gmail.com> > > > > REF:https://edk2.groups.io/g/devel/topic/102310377#110456 > > > > - Add a read after the final PCI Configuration space write > > in RootBridgeIoPciAccess. > > > > - When configuration space is strongly ordered, this ensures > > that program execution cannot continue until the completion > > is received for the previous Cfg-Write, which may have side- > effects. > > > > Cc: Leif Lindholm <quic_llind...@quicinc.com> > > Cc: Ard Biesheuvel <ardb+tianoc...@kernel.org> > > Cc: Sami Mujawar <sami.muja...@arm.com> > > Cc: Jian J Wang <jian.j.w...@intel.com> > > Cc: Liming Gao <gaolim...@byosoft.com.cn> > > Cc: Hao A Wu <hao.a...@intel.com> > > Cc: Ray Ni <ray...@intel.com> > > Cc: Pedro Falcato <pedro.falc...@gmail.com> > > Cc: Michael Brown <mc...@ipxe.org> > > Signed-off-by: Joe Lopez <jlo...@gmail.com> > > --- > > MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c > b/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c > > index 157a0ada80..4bc774b574 100644 > > --- a/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c > > +++ b/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c > > @@ -1238,6 +1238,13 @@ RootBridgeIoPciAccess ( > > } > > } > > > > + // > > + // Perform readback after write to confirm completion was > received for the last write > > + // > > + if (!Read) { > > + PciSegmentRead8 (Address - InStride); > > + } > > + > > return EFI_SUCCESS; > > } > >
PCI Configuration read/write operations are non-posted, so the PCI Configuration write operation should complete without the need for an additional transaction. If you are seeing an issue, then it may be in the implementation of the PciLib/PciSegmentLib that is not guaranteeing this non-posted behavior. Please investigate further and provide details if you think there is an issue in the Pci*Lib implementations. > > (1) I'd like (a) the problem report, and the full reasoning by Ard and > Michael to be captured in the commit message, and (b) *minimally* a > hint > at the possible reordering, and at the PCI spec-based workaround, to > be > placed in the code comment as well. > > (2) This is a significant change; please file a new tianocore BZ about > it. If we include it in the upcoming stable release, the BZ should be > listed here, too: > > https://github.com/tianocore/tianocore.github.io/wiki/EDK-II-Release- > Planning#proposed-features--bug-fixes > > (3) I seem to understand that the outcome of the discusson thus far is > that reading back any config space register should be without side > effects. (In turn, this should be documented in the comment and the > commit message! But, my more important point here is:) > > ... assuming Size is not 1, PciSegmentRead8() will not match the size > of > the most recently performed PciSegmentWriteBuffer(). Is that OK? > > I'm unsure that *any* config space register (especially one in > extended > config space) that is larger than one byte per spec (base PCI spec or > particular device spec) *guarantees* that a 1-byte read at the front > of > that register will behave identically to reading back the entire > register. > > ... What's more, I believe that in the previous discussion, it wasn't > the outcome that any config space register at all can be read back > without side-effects. RootBridgeIoPciAccess() may well read > device-specific registers too, and those can have side-effects upon > read, can't they? > > Laszlo > > > > > -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#110645): https://edk2.groups.io/g/devel/message/110645 Mute This Topic: https://groups.io/mt/102354842/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/leave/9847357/21656/1706620634/xyzzy [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
