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

Jim Quinlan
2024-04-03T21:39:07Z

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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.

-- 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