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

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Date: Wed, 14 Feb 2024 22:35:16 +0100
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: 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 12.02.2024 22:17, Bjorn Helgaas wrote:
> Maybe include the reason in the subject?  "Read back" is literally
> what the diff says.
> 
> On Sat, Feb 10, 2024 at 06:10:06PM +0100, Konrad Dybcio wrote:
>> To ensure write completion, read the PARF_LTSSM register after setting
>> the LTSSM enable bit before polling for "link up".
> 
> The write will obviously complete *some* time; I assume the point is
> that it's important for it to complete before some other event, and it
> would be nice to know why that's important.

Right, that's very much meaningful on non-total-store-ordering
architectures, like arm64, where the CPU receives a store instruction,
but that does not necessarily impact the memory/MMIO state immediately.

Konrad