linux-kernel - Re: [PATCH v2 2/3] PCI: qcom: Read back PARF

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Date: Fri, 15 Mar 2024 17:47:52 +0100
From: Johan Hovold <johan@...nel.org>
To: Konrad Dybcio <konrad.dybcio@...aro.org>
Cc: Bjorn Helgaas <helgaas@...nel.org>,
	Bjorn Andersson <andersson@...nel.org>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	linux-arm-msm@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Johan Hovold <johan+linaro@...nel.org>
Subject: Re: [PATCH v2 2/3] PCI: qcom: Read back PARF_LTSSM register

On Fri, Mar 15, 2024 at 11:16:59AM +0100, Konrad Dybcio wrote:
> On 2/16/24 07:52, Johan Hovold wrote:

> > This makes no sense. As Bjorn already said, you're just polling for the
> > link to come up (for a second). And unless you have something else that
> > depends on the write to have reached the device, there is no need to
> > read it back. It's not going to be cached indefinitely if that's what
> > you fear.
> 
> The point is, if we know that the hardware is expected to return "done"
> within the polling timeout value of receiving the request to do so, we
> are actively taking away an unknown amount of time from that timeout.

We're talking about microseconds, not milliseconds or seconds as you
seem to believe.

> So, if the polling condition becomes true after 980ms, but due to write
> buffering the value reached the PCIe hardware after 21 ms, we're gonna
> hit a timeout. Or under truly extreme circumstances, the polling may
> time out before the write has even arrived at the PCIe hw.

So the write latency is not an issue here.

Johan