linux-kernel - Re: [PATCH v3 3/3] spi: spi-qcom-qspi: Add DMA mode support

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Message-ID: <CAD=FV=VCbcUnf42tK-HV8j=71BXXytxku_0rGjKyhyR3WG4SUw@mail.gmail.com>
Date:   Mon, 17 Apr 2023 07:07:28 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>,
        agross@...nel.org, andersson@...nel.org, konrad.dybcio@...aro.org,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        cros-qcom-dts-watchers@...omium.org, linux-arm-msm@...r.kernel.org,
        linux-spi@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, quic_msavaliy@...cinc.com,
        mka@...omium.org, swboyd@...omium.org, quic_vtanuku@...cinc.com
Subject: Re: [PATCH v3 3/3] spi: spi-qcom-qspi: Add DMA mode support

Hi,

On Mon, Apr 17, 2023 at 5:12 AM Mark Brown <broonie@...nel.org> wrote:
>
> On Fri, Apr 14, 2023 at 03:05:58PM -0700, Doug Anderson wrote:
>
> > Having alignment requirements like this doesn't seem like it should be
> > that unusual, though, and that's why it feels like the logic belongs
> > in the SPI core. In fact, it seems like this is _supposed_ to be
> > handled in the SPI core, but it isn't? In "spi.h" I see
> > "dma_alignment" that claims to be exactly what you need. As far as I
> > can tell, though, the core doesn't use this? ...so I'm kinda confused.
> > As far as I can tell this doesn't do anything and thus anyone setting
> > it today is broken?
>
> SPI consumers should only be providing dmaable buffers.

Ah, I think I see.

1. In "struct spi_transfer" the @tx_buf and @rx_buf are documented to
have "dma-safe memory".

2. On ARM64 anyway, I see "ARCH_DMA_MINALIGN" is 128.

So there is no reason to do any special rules to force alignment to
32-bytes because that's already guaranteed. Presumably that means you
can drop a whole pile of code and things will still work fine.

-Doug