linux-kernel - Re: [PATCH v4 5/5] spi: spi-qcom-qspi: Add DMA mode support

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Message-ID: <CAD=FV=Wt2VWSkSq2FzbtXBrkS06hwizkKMjkgwgKFqAHmcmyNQ@mail.gmail.com>
Date:   Fri, 21 Apr 2023 12:58:57 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>
Cc:     agross@...nel.org, andersson@...nel.org, konrad.dybcio@...aro.org,
        broonie@...nel.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 v4 5/5] spi: spi-qcom-qspi: Add DMA mode support

On Fri, Apr 21, 2023 at 11:21 AM Vijaya Krishna Nivarthi
<quic_vnivarth@...cinc.com> wrote:
>
> >> If we were to have a linked list of descriptors that we can parse and
> >> free, it would require 2 more fields
> >>
> >> this_descriptor_dma - dma address of the current descriptor
> > Isn't that exactly the same value as "data_address"? Sure,
> > "data_address" is a u32 and the DMA address is 64-bits, but elsewhere
> > in the code you already rely on the fact that the upper bits of the
> > DMA address are 0 when you do:
>
>
> No; data_address is the dma_address mapped to the xfer buffer.
>
> This is provided by spi framework and retrieved from sgl.
>
> "this_descriptor" would be the dma_address of the current cmd_descriptor.
>
> this is returned from dma_pool_alloc()
>
> this would be required for freeing.

Oh! Of course, that's right. So you are correct, you'd need to add
"this_descriptor_dma", but not the virtual address since that would be
the same as the address of the structure via the list_node_t. I guess
I won't insist on using a linked list even though it seems more
elegant to me. In the very least it should fall back to PIO if the
array isn't enough and if we need to change it later we always can.

-Doug