linux-kernel - Re: [PATCH 3/3] i2c: qup: Vote for interconnect bandwidth to DRAM

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Date:   Tue, 28 Nov 2023 20:17:57 +0100
From:   Andi Shyti <andi.shyti@...nel.org>
To:     Stephan Gerhold <stephan.gerhold@...nkonzept.com>
Cc:     Wolfram Sang <wsa@...nel.org>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        linux-arm-msm@...r.kernel.org, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] i2c: qup: Vote for interconnect bandwidth to DRAM

Hi Stephan,

On Tue, Nov 28, 2023 at 10:48:37AM +0100, Stephan Gerhold wrote:
> When the I2C QUP controller is used together with a DMA engine it needs
> to vote for the interconnect path to the DRAM. Otherwise it may be
> unable to access the memory quickly enough.
> 
> The requested peak bandwidth is dependent on the I2C core clock.
> 
> To avoid sending votes too often the bandwidth is always requested when
> a DMA transfer starts, but dropped only on runtime suspend. Runtime
> suspend should only happen if no transfer is active. After resumption we
> can defer the next vote until the first DMA transfer actually happens.
> 
> The implementation is largely identical to the one introduced for
> spi-qup in commit ecdaa9473019 ("spi: qup: Vote for interconnect
> bandwidth to DRAM") since both drivers represent the same hardware
> block.
> 
> Signed-off-by: Stephan Gerhold <stephan.gerhold@...nkonzept.com>

the patch looks good to me.

> ---
> The bandwidth calculation is taken over from Qualcomm's
> downstream/vendor driver [1]. Due to lack of documentation about the
> interconnect setup/behavior I cannot say exactly if this is right.
> Unfortunately, this is not implemented very consistently downstream...
> 
> [1]: https://git.codelinaro.org/clo/la/kernel/msm-3.10/-/commit/67174e2624ea64814231e7e1e4af83fd882302c6

Krzysztof, any chance you can help here?

Thanks,
Andi