On Wed, Jan 11, 2023 at 03:27:51PM -0800, Sathyanarayanan Kuppuswamy wrote: > 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 > > 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 agree, it seems pointless. > > 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. Ah, right, and I think I was even part of the conversation when this was added :) I'm not sure I would make the same choice today, though. IMHO it's kind of hard to defend choosing performance over data integrity. If a platform really wants to sacrifice integrity for performance, it could retain control of AER, and after Vidya's patch, Linux will leave the ECRC configuration alone. Straw-man: If Linux owns AER and ECRC is supported, enable ECRC by default. Retain "ecrc=off" to turn it off, but drop a note in dmesg and taint the kernel. Bjorn
