linux-kernel - Re: [PATCH] arm64: dts: qcom: sc8280xp: fix PCIe DMA coherency

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Message-ID: <Y4DkA6Tbf9exhnL7@hovoldconsulting.com>
Date:   Fri, 25 Nov 2022 16:49:23 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: qcom: sc8280xp: fix PCIe DMA coherency

On Fri, Nov 25, 2022 at 08:23:36PM +0530, Manivannan Sadhasivam wrote:
> On Fri, Nov 25, 2022 at 03:43:59PM +0100, Johan Hovold wrote:
> > On Fri, Nov 25, 2022 at 07:56:25PM +0530, Manivannan Sadhasivam wrote:
> > > On Thu, Nov 24, 2022 at 03:25:01PM +0100, Johan Hovold wrote:

> > I never claimed it would fix the problem, I explicitly wrote that it
> > made it less likely to occur (to the point where my reproducer no longer
> > triggers).
> 
> > Increasing the buffer sizes to two pages to force CMA allocation also appears
> > to make the problem go away.
> 
> The "go away" part sounded like a claim to me and hence I added the statement.
> But no worries :)

Hopefully it's clear enough if you also read the preceding sentence
(with emphasis added):

  presumably as these queues are small enough to not be allocated using
  CMA which in turn make them *more likely to be cached* (e.g. due to
  accesses to nearby pages through the cacheable linear map).

Johan