linux-kernel - Re: [PATCH 13/31] nds32: DMA mapping API

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Date:   Wed, 8 Nov 2017 10:09:31 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Greentime Hu <green.hu@...il.com>
Cc:     greentime@...estech.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH 13/31] nds32: DMA mapping API

On Wed, Nov 8, 2017 at 6:55 AM, Greentime Hu <green.hu@...il.com> wrote:

> +static void consistent_sync(void *vaddr, size_t size, int direction)
> +{
> +       unsigned long start = (unsigned long)vaddr;
> +       unsigned long end = start + size;
> +
> +       switch (direction) {
> +       case DMA_FROM_DEVICE:   /* invalidate only */
> +               cpu_dma_inval_range(start, end);
> +               break;
> +       case DMA_TO_DEVICE:     /* writeback only */
> +               cpu_dma_wb_range(start, end);
> +               break;
> +       case DMA_BIDIRECTIONAL: /* writeback and invalidate */
> +               cpu_dma_wbinval_range(start, end);
> +               break;
> +       default:
> +               BUG();
> +       }
> +}

> +
> +static void
> +nds32_dma_sync_single_for_cpu(struct device *dev, dma_addr_t handle,
> +                             size_t size, enum dma_data_direction dir)
> +{
> +       consistent_sync((void *)dma_to_virt(dev, handle), size, dir);
> +}
> +
> +static void
> +nds32_dma_sync_single_for_device(struct device *dev, dma_addr_t handle,
> +                                size_t size, enum dma_data_direction dir)
> +{
> +       consistent_sync((void *)dma_to_virt(dev, handle), size, dir);
> +}

You do the same cache operations for _to_cpu and _to_device, which
usually works,
but is more expensive than you need. It's better to take the ownership into
account and only do what you need.

     Arnd