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

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Date:   Fri, 21 Jan 2022 06:55:11 +0000
From:   Lh Kuo 郭力豪 <lh.Kuo@...plus.com>
To:     Andy Shevchenko <andy.shevchenko@...il.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

> > > > +       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.