Re: [edk2-devel] [PATCH] MdeModulePkg/PciHostBridgeDxe: Add readback after final Cfg-Write

["Michael D Kinney" <michael.d.kinney@intel.com>]

> -----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; jlotwo@gmail.com
> Cc: Leif Lindholm <quic_llindhol@quicinc.com>; Ard Biesheuvel
> <ardb+tianocore@kernel.org>; Sami Mujawar <sami.mujawar@arm.com>;
> Wang, Jian J <jian.j.wang@intel.com>; Gao, Liming
> <gaoliming@byosoft.com.cn>; Wu, Hao A <hao.a.wu@intel.com>; Ni, Ray
> <ray.ni@intel.com>; Pedro Falcato <pedro.falcato@gmail.com>; Michael
> Brown <mcb30@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 <jlotwo@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_llindhol@quicinc.com>
> > Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> > Cc: Sami Mujawar <sami.mujawar@arm.com>
> > Cc: Jian J Wang <jian.j.wang@intel.com>
> > Cc: Liming Gao <gaoliming@byosoft.com.cn>
> > Cc: Hao A Wu <hao.a.wu@intel.com>
> > Cc: Ray Ni <ray.ni@intel.com>
> > Cc: Pedro Falcato <pedro.falcato@gmail.com>
> > Cc: Michael Brown <mcb30@ipxe.org>
> > Signed-off-by: Joe Lopez <jlotwo@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
> 
> 
> 
> 
> 



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