linux-kernel - Re: [PATCH v6 1/2] spi: Add spi driver for Sunplus SP7021

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Message-ID: <CAHp75VcmFPCC0kDxOma6gunwFRf-eXEr6+ZxQs1dt5GH2quT4Q@mail.gmail.com>
Date:   Fri, 21 Jan 2022 12:09:15 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Lh Kuo 郭力豪 <lh.Kuo@...plus.com>
Cc:     Li-hao Kuo <lhjeff911@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Mark Brown <broonie@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wells Lu 呂芳騰 <wells.lu@...plus.com>
Subject: Re: [PATCH v6 1/2] spi: Add spi driver for Sunplus SP7021

On Fri, Jan 21, 2022 at 8:54 AM Lh Kuo 郭力豪 <lh.Kuo@...plus.com> wrote:
>
> > > > > +       if (xfer->tx_buf)
> > > > > +               dma_unmap_single(dev, xfer->tx_dma, xfer->len, DMA_TO_DEVICE);
> > > > > +       if (xfer->rx_buf)
> > > > > +               dma_unmap_single(dev, xfer->rx_dma, xfer->len,
> > > > > + DMA_FROM_DEVICE);
> > > >
> > > > Why can't you use SPI core DMA mapping code?
> > >
> > > I didn't find the SPI core DMA mapping code for single maping.
> > > The method currently used is the general DMA single-map code usage method.
> >
> > Why do you need single page mapping?
> > What's wrong with SG mapping that SPI core provides?
>
> SP7021 SPI slave dma only supports single dma in one trigger.
> It is not suitable for using SG mapping.
> If the length of the transfer is larger than the length of the SG-mapping,
> Slave-mode will get error in the transfer.

Same story for SPI DesignWare on Intel Medfield, where no SG transfers
are supported by hardware. Nevertheless, the DMA driver takes care of
this and on each interrupt recharges a channel to continue. Why can't
the same be implemented here?


-- 
With Best Regards,
Andy Shevchenko