Re: [PATCH v9 0/4] PCI: brcmstb: Configure appropriate HW CLKREQ# mode

From: Bjorn Helgaas
Date: Mon May 06 2024 - 18:32:03 EST


On Tue, Apr 30, 2024 at 05:02:45PM -0400, Jim Quinlan wrote:
> On Wed, Apr 3, 2024 at 5:39 PM Jim Quinlan <james.quinlan@xxxxxxxxxxxx> wrote:
> >
> > v9 -- v8 was setting an internal bus timeout to accomodate large L1 exit
> > latencies. After meeting the PCIe HW team it was revealed that the
> > HW default timeout value was set low for the purposes of HW debugging
> > convenience; for nominal operation it needs to be set to a higher
> > value independent of this submission's purpose. This is now a
> > separate commit.
>
> Bjorn,
>
> Did you have some time to look at this? Do you have any comments or questions?

Sorry, I didn't realize you were waiting on me. I think Krzysztof W.
will ultimately take care of these.

I have some minor comments but overall I'm fine with this.

> > -- With v8, Bjorne asked what was preventing a device from exceeding the
> > time required for the above internal bus timeout. The answer to this
> > is for us to set the endpoints' max latency {no-,}snoop value to
> > something below this internal timeout value. If the endpoint
> > respects this value as it should, it will not send an LTR request
> > with a larger latency value and not put itself in a situation
> > that requires more latency than is possible for the platform.
> >
> > Typically, ACPI or FW sets these max latency values. In most of our
> > systems we do not have this happening so it is up to the RC driver to
> > set these values in the endpoint devices. If the endpoints already
> > have non-zero values that are lower than what we are setting, we let
> > them be, as it is possible ACPI or FW set them and knows something
> > that we do not.
> >
> > -- The "clkreq" commit has only been changed to remove the code that was
> > setting the timeout value, as this code is now its own commit.
> >
> > v8 -- Un-advertise L1SS capability when in "no-l1ss" mode (Bjorn)
> > -- Squashed last two commits of v7 (Bjorn)
> > -- Fix DT binding description text wrapping (Bjorn)
> > -- Fix incorrect Spec reference (Bjorn)
> > s/PCIe Spec/PCIe Express Mini CEM 2.1 specification/
> > -- Text substitutions (Bjorn)
> > s/WRT/With respect to/
> > s/Tclron/T_CLRon/
> >
> > v7 -- Manivannan Sadhasivam suggested (a) making the property look like a
> > network phy-mode and (b) keeping the code simple (not counting clkreq
> > signal appearances, un-advertising capabilites, etc). This is
> > what I have done. The property is now "brcm,clkreq-mode" and
> > the values may be one of "safe", "default", and "no-l1ss". The
> > default setting is to employ the most capable power savings mode.
> >
> > v6 -- No code has been changed.
> > -- Changed commit subject and comment in "#PERST" commit (Bjorn, Cyril)
> > -- Changed sign-off and author email address for all commits.
> > This was due to a change in Broadcom's upstreaming policy.
> >
> > v5 -- Remove DT property "brcm,completion-timeout-us" from
> > "DT bindings" commit. Although this error may be reported
> > as a completion timeout, its cause was traced to an
> > internal bus timeout which may occur even when there is
> > no PCIe access being processed. We set a timeout of four
> > seconds only if we are operating in "L1SS CLKREQ#" mode.
> > -- Correct CEM 2.0 reference provided by HW engineer,
> > s/3.2.5.2.5/3.2.5.2.2/ (Bjorn)
> > -- Add newline to dev_info() string (Stefan)
> > -- Change variable rval to unsigned (Stefan)
> > -- s/implementaion/implementation/ (Bjorn)
> > -- s/superpowersave/powersupersave/ (Bjorn)
> > -- Slightly modify message on "PERST#" commit.
> > -- Rebase to torvalds master
> >
> > v4 -- New commit that asserts PERST# for 2711/RPi SOCs at PCIe RC
> > driver probe() time. This is done in Raspian Linux and its
> > absence may be the cause of a failing test case.
> > -- New commit that removes stale comment.
> >
> > v3 -- Rewrote commit msgs and comments refering panics if L1SS
> > is enabled/disabled; the code snippet that unadvertises L1SS
> > eliminates the panic scenario. (Bjorn)
> > -- Add reference for "400ns of CLKREQ# assertion" blurb (Bjorn)
> > -- Put binding names in DT commit Subject (Bjorn)
> > -- Add a verb to a commit's subject line (Bjorn)
> > -- s/accomodat(\w+)/accommodat$1/g (Bjorn)
> > -- Rewrote commit msgs and comments refering panics if L1SS
> > is enabled/disabled; the code snippet that unadvertises L1SS
> > eliminates the panic scenario. (Bjorn)
> >
> > v2 -- Changed binding property 'brcm,completion-timeout-msec' to
> > 'brcm,completion-timeout-us'. (StefanW for standard suffix).
> > -- Warn when clamping timeout value, and include clamped
> > region in message. Also add min and max in YAML. (StefanW)
> > -- Qualify description of "brcm,completion-timeout-us" so that
> > it refers to PCIe transactions. (StefanW)
> > -- Remvove mention of Linux specifics in binding description. (StefanW)
> > -- s/clkreq#/CLKREQ#/g (Bjorn)
> > -- Refactor completion-timeout-us code to compare max and min to
> > value given by the property (as opposed to the computed value).
> >
> > v1 -- The current driver assumes the downstream devices can
> > provide CLKREQ# for ASPM. These commits accomodate devices
> > w/ or w/o clkreq# and also handle L1SS-capable devices.
> >
> > -- The Raspian Linux folks have already been using a PCIe RC
> > property "brcm,enable-l1ss". These commits use the same
> > property, in a backward-compatible manner, and the implementaion
> > adds more detail and also automatically identifies devices w/o
> > a clkreq# signal, i.e. most devices plugged into an RPi CM4
> > IO board.
> >
> > Jim Quinlan (4):
> > dt-bindings: PCI: brcmstb: Add property "brcm,clkreq-mode"
> > PCI: brcmstb: Set reasonable value for internal bus timeout
> > PCI: brcmstb: Set downstream maximum {no-}snoop LTR values
> > PCI: brcmstb: Configure HW CLKREQ# mode appropriate for downstream
> > device
> >
> > .../bindings/pci/brcm,stb-pcie.yaml | 18 ++
> > drivers/pci/controller/pcie-brcmstb.c | 161 +++++++++++++++++-
> > 2 files changed, 170 insertions(+), 9 deletions(-)
> >
> >
> > base-commit: 9f8413c4a66f2fb776d3dc3c9ed20bf435eb305e
> > --
> > 2.17.1
> >