linux-kernel - Re: [PATCH] PCI: qcom-ep: Do not enable resources during probe()

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Message-ID: <20240822151658.GA305162@bhelgaas>
Date: Thu, 22 Aug 2024 10:16:58 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: Krzysztof Wilczyński <kw@...ux.com>,
	lpieralisi@...nel.org, robh@...nel.org, bhelgaas@...gle.com,
	linux-arm-msm@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: qcom-ep: Do not enable resources during probe()

On Thu, Aug 22, 2024 at 12:18:23PM +0530, Manivannan Sadhasivam wrote:
> On Wed, Aug 21, 2024 at 05:56:18PM -0500, Bjorn Helgaas wrote:
> ...

> > Although I do have the question of what happens if the RC deasserts
> > PERST# before qcom-ep is loaded.  We probably don't execute
> > qcom_pcie_perst_deassert() in that case, so how does the init happen?
> 
> PERST# is a level trigger signal. So even if the host has asserted
> it before EP booted, the level will stay low and ep will detect it
> while booting.

The PERST# signal itself is definitely level oriented.

I'm still skeptical about the *interrupt* from the PCIe controller
being level-triggered, as I mentioned here:
https://lore.kernel.org/r/20240815224735.GA57931@bhelgaas

tegra194 is also dwc-based and has a similar PERST# interrupt but it's
edge-triggered (tegra_pcie_ep_pex_rst_irq()), which I think is a
cleaner implementation.  Then you don't have to remember the current
state, switch between high and low trigger, worry about races and
missing a pulse, etc.

> If it is an edge trigger signal, then ep wouldn't be able to catch
> it as you suspected.

Bjorn