From: Bjorn Helgaas <helgaas@kernel.org>
To: Jim Quinlan <james.quinlan@broadcom.com>
Cc: linux-pci@vger.kernel.org,
"Nicolas Saenz Julienne" <nsaenz@kernel.org>,
"Bjorn Helgaas" <bhelgaas@google.com>,
"Lorenzo Pieralisi" <lorenzo.pieralisi@arm.com>,
"Cyril Brulebois" <kibi@debian.org>,
"Phil Elwell" <phil@raspberrypi.com>,
bcm-kernel-feedback-list@broadcom.com,
"Florian Fainelli" <florian.fainelli@broadcom.com>,
"Jim Quinlan" <jim2101024@gmail.com>,
"Lorenzo Pieralisi" <lpieralisi@kernel.org>,
"Krzysztof Wilczyński" <kw@linux.com>,
"Rob Herring" <robh@kernel.org>,
"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE"
<linux-rpi-kernel@lists.infradead.org>,
"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE"
<linux-arm-kernel@lists.infradead.org>,
"open list" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v9 4/4] PCI: brcmstb: Configure HW CLKREQ# mode appropriate for downstream device
Date: Wed, 8 May 2024 18:33:02 -0500 [thread overview]
Message-ID: <20240508233302.GA1792067@bhelgaas> (raw)
In-Reply-To: <CA+-6iNxEZm=axRAeeAKwxemjEdjjmJYTUs8nThp_NDohXcV5Jg@mail.gmail.com>
On Wed, May 08, 2024 at 01:55:24PM -0400, Jim Quinlan wrote:
> On Mon, May 6, 2024 at 7:20 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> ...
> > As a user, how do I determine which setting to use?
>
> Using the "safe" mode will always work. In fact I considered making
> this the default mode.
> As I said, I cannot enumerate all of the reasons why one mode works
> and one does not for a particular device+board+connector combo. The
> HW folks have not really been forthcoming on the reasons as well.
>
> > Trial and error? If so, how do I identify the errors?
>
> Either PCIe link-up is not happening, or it is happening but the
> device driver is non-functional and boot typically hangs.
What I'm hearing is that it's trial and error.
If we can't tell users how to figure out which mode to use, I think we
have to explicitly say "try the modes in this order until you find one
that works."
That sucks, but if it's all we can do, I guess we don't have much
choice, and we should just own up to it.
There's no point in telling users "if your card drives CLKREQ# use X,
but if not and it can tolerate out-of-spec T_CLRon timing, use Y"
because nobody knows how to figure that out.
And we can say which features are enabled in each mode so they aren't
surprised, e.g., something like this:
"default" -- The Root Port supports ASPM L0s, L1, L1 Substates, and
Clock Power Management. This provides the best power savings but
some devices may not work correctly because the Root Port doesn't
comply with T_CLRon timing required for PCIe Mini Cards [1].
"no-l1ss" -- The Root Port supports ASPM L0s, L1 (but not L1
Substates), and Clock Power Management. [I assume there's some
other Root Port defect that causes issues with some devices in
this mode; I dunno. If we don't know exactly what it is, I guess
we can't really say anything.]
"safe" -- The Root Port supports ASPM L0, L1, L1 Substates, but not
Clock Power Management. All devices should work in this mode.
[1] PCIe Mini CEM r2.1, sec 3.2.5.2.2
(I'm not sure which features are *actually* enabled in each mode, I
just guessed.)
Bjorn
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next prev parent reply other threads:[~2024-05-08 23:33 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-03 21:38 [PATCH v9 0/4] PCI: brcmstb: Configure appropriate HW CLKREQ# mode Jim Quinlan
2024-04-03 21:38 ` [PATCH v9 1/4] dt-bindings: PCI: brcmstb: Add property "brcm,clkreq-mode" Jim Quinlan
2024-04-03 21:38 ` [PATCH v9 2/4] PCI: brcmstb: Set reasonable value for internal bus timeout Jim Quinlan
2024-05-06 22:45 ` Bjorn Helgaas
2024-04-03 21:39 ` [PATCH v9 3/4] PCI: brcmstb: Set downstream maximum {no-}snoop LTR values Jim Quinlan
2024-04-03 21:39 ` [PATCH v9 4/4] PCI: brcmstb: Configure HW CLKREQ# mode appropriate for downstream device Jim Quinlan
2024-05-06 23:20 ` Bjorn Helgaas
2024-05-08 17:55 ` Jim Quinlan
2024-05-08 23:33 ` Bjorn Helgaas [this message]
2024-04-04 20:01 ` [PATCH v9 0/4] PCI: brcmstb: Configure appropriate HW CLKREQ# mode Cyril Brulebois
2024-04-30 21:02 ` Jim Quinlan
2024-05-06 22:31 ` Bjorn Helgaas
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