Re: [PATCH 1/1] PCI: Fix Extend ACS configurability

Tushar Dave
2025-01-06T20:34:13Z

On 1/2/25 10:40, Jason Gunthorpe wrote:
> On Fri, Dec 13, 2024 at 12:29:42PM -0800, Tushar Dave wrote:
>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index dc663c0ca670..fc1c37910d1c 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -4654,11 +4654,10 @@
>> Format:
>> <ACS flags>@<pci_dev>[; ...]
>> Specify one or more PCI devices (in the format
>> - specified above) optionally prepended with flags
>> - and separated by semicolons. The respective
>> - capabilities will be enabled, disabled or
>> - unchanged based on what is specified in
>> - flags.
>> + specified above) prepended with flags and separated
>> + by semicolons. The respective capabilities will be
>> + enabled, disabled or unchanged based on what is
>> + specified in flags.
>>
>> ACS Flags is defined as follows:
>> bit-0 : ACS Source Validation
>> @@ -4673,7 +4672,7 @@
>> '1' – force enabled
>> 'x' – unchanged
>> For example,
>> - pci=config_acs=10x
>> + pci=config_acs=10x@pci:0:0
>> would configure all devices that support
>> ACS to enable P2P Request Redirect, disable
>> Translation Blocking, and leave Source
>
> Is this an unrelated change? The format of the command line shouldn't
> be changed to fix the described bug, why is the documentation changed?

The documentation as it is (i.e. without my patch), is not correct.

IOW, config_acs parameter does require flags and it is not optional. Without
flags it results into "ACS Flags missing". Therefore I remove word "optionally"
from the documentation text.

Secondly, the syntax in the example 'pci=config_acs=10x' is incorrect. The
correct syntax should be 'pci=config_acs=10x@pci:0:0' that would configure all
devices that support ACS to enable P2P Request Redirect, disable Translation
Blocking, and leave Source Validation unchanged from whatever power-up or
firmware set it to.

>
>> static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
>> - const char *p, u16 mask, u16 flags)
>> + const char *p, const u16 acs_mask, const u16 acs_flags)
>> {
>> + u16 flags = acs_flags;
>> + u16 mask = acs_mask;
>> char *delimit;
>> int ret = 0;
>>
>> @@ -964,7 +965,7 @@ static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
>> return;
>>
>> while (*p) {
>> - if (!mask) {
>> + if (!acs_mask) {
>> /* Check for ACS flags */
>> delimit = strstr(p, "@");
>> if (delimit) {
>> @@ -972,6 +973,8 @@ static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
>> u32 shift = 0;
>>
>> end = delimit - p - 1;
>> + mask = 0;
>> + flags = 0;
>>
>> while (end > -1) {
>> if (*(p + end) == '0') {
>
> This function the entire fix, right? Because the routine was
> clobbering acs_mask as it processed the earlier devices?

yes, that is correct.

>
>> @@ -1028,10 +1031,10 @@ static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
>>
>> pci_dbg(dev, "ACS mask = %#06x\n", mask);
>> pci_dbg(dev, "ACS flags = %#06x\n", flags);
>> + pci_dbg(dev, "ACS control = %#06x\n", caps->ctrl);
>>
>> - /* If mask is 0 then we copy the bit from the firmware setting. */
>> - caps->ctrl = (caps->ctrl & ~mask) | (caps->fw_ctrl & mask);
>> - caps->ctrl |= flags;
>> + caps->ctrl &= ~mask;
>> + caps->ctrl |= (flags & mask);
>
> And why delete fw_ctrl? Doesn't that break the unchanged
> functionality?

No, it does not break the unchanged functionality. I removed it because it is
not needed after my fix.

If it helps, using 'config_acs' the code only allows to configures the lower 7
bits of ACS ctrl for the specified PCI device(s).
The bits other than the lower 7 bits of ACS ctrl remain unchanged.
The bits specified with 'x' or 'X' that are within the 7 lower bits remain
unchanged. Trying to configure bits other than lower 7 bits generates an error
message "Invalid ACS flags specified"

-Tushar
>
> Jason