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SubjectRe: [PATCH V1] PCI/AER: Configure ECRC only AER is native
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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@hp.com>
> 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@nvidia.com>
>>>> ---
>>>> 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

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