Re: [PATCH v6 3/6] PCI: qcom: Add missing icc bandwidth vote for cpu to PCIe path

From: Konrad Dybcio <konrad.dybcio@linaro.org>
To: Johan Hovold <johan@kernel.org>
Cc: "Bryan O'Donoghue" <bryan.odonoghue@linaro.org>,
	"Krishna chaitanya chundru" <quic_krichai@quicinc.com>,
	"Bjorn Andersson" <andersson@kernel.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Lorenzo Pieralisi" <lpieralisi@kernel.org>,
	"Krzysztof Wilczyński" <kw@linux.com>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Conor Dooley" <conor+dt@kernel.org>,
	"Manivannan Sadhasivam" <mani@kernel.org>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Johan Hovold" <johan+linaro@kernel.org>,
	"Brian Masney" <bmasney@redhat.com>,
	"Georgi Djakov" <djakov@kernel.org>,
	linux-arm-msm@vger.kernel.org, vireshk@kernel.org,
	quic_vbadigan@quicinc.com, quic_skananth@quicinc.com,
	quic_nitegupt@quicinc.com, linux-pci@vger.kernel.org,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH v6 3/6] PCI: qcom: Add missing icc bandwidth vote for cpu to PCIe path
Date: Wed, 17 Jan 2024 10:13:15 +0100	[thread overview]
Message-ID: <ca4f3f38-f0e5-4d10-b9eb-155f6ffbc57c@linaro.org> (raw)
In-Reply-To: <ZaZf4EyX5oADXG9N@hovoldconsulting.com>

On 1/16/24 11:52, Johan Hovold wrote:
> On Fri, Jan 12, 2024 at 11:33:15PM +0100, Konrad Dybcio wrote:
>> On 12.01.2024 16:17, Bryan O'Donoghue wrote:
>>> On 12/01/2024 14:22, Krishna chaitanya chundru wrote:
>>>> CPU-PCIe path consits for registers PCIe BAR space, config space.
>>>> As there is less access on this path compared to pcie to mem path
>>>> add minimum vote i.e GEN1x1 bandwidth always.
>>>>
>>>> In suspend remove the cpu vote after register space access is done.
>>>>
>>>> Fixes: c4860af88d0c ("PCI: qcom: Add basic interconnect support")
>>>
>>> If this patch is a Fixes then don't you need the accompanying dts change as a parallel Fixes too ?
>>>
>>> i.e. without the dts update - you won't have the nodes in the dts to consume => applying this code to the stable kernel absent the dts will result in no functional change and therefore no bugfix.
>>
>> The Fixes tag denotes a bug fix, its use for backport autosel is just
>> a nice "coincidence".
>>
>> Fixing a lack of a required icc path and having to rely on BL leftovers
>> / keepalive bus settings is definitely worth this tag in my eyes.
> 
> An incomplete implementation can sometimes be considered a bug, but not
> always. If this is needed to enable a new use case, then it's hard to
> argue that the original omission was a bug.
> 
> And as I just mentioned to Krishna, the above Fixes tag is not correct
> as that commit did not *introduce* any issue. It solved the bit that was
> strictly needed for sc8280xp, but now it seems you may need something
> more for an even newer platform (even if no details and motivation was
> included in the commit message as it should have been).

The PCIe hardware seems to be piggybacking off of others' bus
bandwidth requests and I think it's just been pure luck that
it didn't simply refuse to work on previous generations.

So indeed, the commit message seems incomplete in explaining
where the problem lies

Konrad