Re: [PATCH v8 2/2] PCI: brcmstb: Configure HW CLKREQ# mode appropriate for downstream device

[Jim Quinlan]

On Thu, Jan 11, 2024 at 3:54 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Thu, Jan 11, 2024 at 01:20:48PM -0500, Jim Quinlan wrote:
> > On Thu, Jan 11, 2024 at 12:28 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > > On Mon, Nov 13, 2023 at 01:56:06PM -0500, Jim Quinlan wrote:
>
> > > > Previously, this driver always set the mode to "no-l1ss", as almost all
> > > > STB/CM boards operate in this mode.  But now there is interest in
> > > > activating L1SS power savings from STB/CM customers, which requires "aspm"
> > > > mode.
> > >
> > > I think this should read "default" mode, not "aspm" mode, since "aspm"
> > > is not a mode implemented by this patch, right?
> >
> > Correct.
>
> Thanks, I changed that locally.
>
> > > > In addition, a bug was filed for RPi4 CM platform because most
> > > > devices did not work in "no-l1ss" mode.
> > >
> > > I think this refers to bug 217276, mentioned below?
> >
> > I guess you are saying I should put a footnote marker there.
>
> I added a hint here.
>
> > > > Note: Since L1 substates are now possible, a modification was made
> > > > regarding an internal bus timeout: During long periods of the PCIe RC HW
> > > > being in an L1SS sleep state, there may be a timeout on an internal bus
> > > > access, even though there may not be any PCIe access involved.  Such a
> > > > timeout will cause a subsequent CPU abort.
> > >
> > > This sounds scary.  If a NIC is put in L1.2, does this mean will we
> > > see this CPU abort if there's no traffic for a long time?  What is
> > > needed to avoid the CPU abort?
> >
> > I don't think this happens in normal practice as there are a slew
> > of low-level TLPs and LTR messages that are sent on a regular
> > basis.
>
> OK, I'll have to take your word for this.  I don't know enough about
> PCIe to know what sort of periodic transmissions are required when a
> device is idle.
>
> LTR messages are required when endpoint service requirements change,
> but I wouldn't expect those if the device is idle.
>
> > The only time this timeout occured is when  a major customer
> > was doing a hack: IIRC, their endpoint device has to reboot itself
> > after link-up and driver probe,  so it goes into L1.2 to execute
> > this to reboot and while doing so the connection is completely
> > silent.
>
> > > What does this mean for users?  L1SS is designed for long periods of
> > > the device being idle, so this leaves me feeling that using L1SS is
> > > unsafe in general.  Hopefully this impression is unwarranted, and all
> > > we need is some clarification here.
> >
> > I don't think it will affect most users, if any.
>
> I'll try to get this into -next today or tomorrow.

Bjorn, you are right -- I need to cajole our PCIe HW team to tell me
why this timeout can never
happen and/or why it is not a bug.
Until then,
Jim Quinlan
Broadcom STB/CM


If
>
> Bjorn

Attachment: smime.p7s
Description: S/MIME Cryptographic Signature