Re: [PATCH V1] PCI/AER: Configure ECRC only AER is native

[Vidya Sagar]

On 1/12/2023 4:57 AM, Sathyanarayanan Kuppuswamy wrote:
> External email: Use caution opening links or attachments
>
>
> Hi,
>
> On 1/11/23 3:10 PM, Bjorn Helgaas wrote:
> > On Wed, Jan 11, 2023 at 01:42:21PM -0800, Sathyanarayanan Kuppuswamy wrote:
> > > On 1/11/23 12:31 PM, Vidya Sagar wrote:
> > > > As the ECRC configuration bits are part of AER registers, configure
> > > > ECRC only if AER is natively owned by the kernel.
> > >
> > > ecrc command line option takes "bios/on/off" as possible options. It
> > > does not clarify whether "on/off" choices can only be used if AER is
> > > owned by OS or it can override the ownership of ECRC configuration
> > > similar to pcie_ports=native option. Maybe that needs to be clarified.
> >
> > Good point, what do you think of an update like this:
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 6cfa6e3996cf..f7b40a439194 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4296,7 +4296,9 @@
> >                                specified, e.g., 12@pci:8086:9c22:103c:198f
> >                                for 4096-byte alignment.
> >                ecrc=           Enable/disable PCIe ECRC (transaction layer
> > -                             end-to-end CRC checking).
> > +                             end-to-end CRC checking).  Only effective
> > +                             if OS has native AER control (either granted by
> > +                             ACPI _OSC or forced via "pcie_ports=native").
> >                                bios: Use BIOS/firmware settings. This is the
> >                                the default.
> >                                off: Turn ECRC off
I'm also fine with this change. I'll take it in V2.
> Looks fine. But do we even need "bios" option? Since it is the default
> value, I am not sure why we need to list that as an option again. IMO
> this could be removed.
I think we still need bios option. For example, consider a system where 
BIOS needs to keep ECRC enabled for integrity reasons but if kernel 
doesn't want it for perf reasons, then, kernel can always use 'ecrc=off' 
option.
> > I don't know whether the "ecrc=" parameter is really needed.  If we
> > were adding it today, I would ask "why not enable ECRC wherever it is
> > supported?"  If there are devices where it's broken, we could always
> > add quirks to disable it on a case-by-case basis.
>
> Checking the original patch which added it, it looks like the intention
> is to give option to boost performance over integrity.
>
> commit 43c16408842b0eeb367c23a6fa540ce69f99e347
> Author: Andrew Patterson <andrew.patterson@xxxxxx>
> Date:   Wed Apr 22 16:52:09 2009 -0600
>
>      PCI: Add support for turning PCIe ECRC on or off
>
>      Adds support for PCI Express transaction layer end-to-end CRC checking
>      (ECRC).  This patch will enable/disable ECRC checking by setting/clearing
>      the ECRC Check Enable and/or ECRC Generation Enable bits for devices that
>      support ECRC.
>
>      The ECRC setting is controlled by the "pci=ecrc=<policy>" command-line
>      option. If this option is not set or is set to 'bios", the enable and
>      generation bits are left in whatever state that firmware/BIOS set them to.
>      The "off" setting turns them off, and the "on" option turns them on (if the
>      device supports it).
>
>      Turning ECRC on or off can be a data integrity versus performance
>      tradeoff.  In theory, turning it on will catch more data errors, turning
>      it off means possibly better performance since CRC does not need to be
>      calculated by the PCIe hardware and packet sizes are reduced.
>
>
> > But I think the patch below is the right thing to do for now.  Vidya,
>
> Agree.
>
> > did you trip over an issue because of this, e.g., a conflict between
> > firmware use of AER and Linux use of it?  If so, maybe we could
> > mention a symptom on the commit log.  But my guess is you probably
> > found this by inspection.
Not really. I was just checking when does kernel touch ECRC settings and 
happened to find this where it configures ECRC irrespective of its 
ownership of AER registers.

> > Bjorn
> >
> > > > Signed-off-by: Vidya Sagar <vidyas@xxxxxxxxxx>
> > > > ---
> > > >   drivers/pci/pcie/aer.c | 3 +++
> > > >   1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/drivers/pci/pcie/aer.c b/drivers/pci/pcie/aer.c
> > > > index e2d8a74f83c3..730b47bdcdef 100644
> > > > --- a/drivers/pci/pcie/aer.c
> > > > +++ b/drivers/pci/pcie/aer.c
> > > > @@ -184,6 +184,9 @@ static int disable_ecrc_checking(struct pci_dev *dev)
> > > >    */
> > > >   void pcie_set_ecrc_checking(struct pci_dev *dev)
> > > >   {
> > > > +   if (!pcie_aer_is_native(dev))
> > > > +           return;
> > > > +
> > > >      switch (ecrc_policy) {
> > > >      case ECRC_POLICY_DEFAULT:
> > > >              return;
> > >
> > > --
> > > Sathyanarayanan Kuppuswamy
> > > Linux Kernel Developer
>
> --
> Sathyanarayanan Kuppuswamy
> Linux Kernel Developer