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

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Date: Wed, 27 Mar 2024 20:37:43 +0100
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Johan Hovold <johan@...nel.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 15.03.2024 5:47 PM, Johan Hovold wrote:
> 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.

Right, I'm willing to believe the CPU will kick the can down the road
for this long. I'll drop this.

Konrad