Re: [PATCH v2 3/5] spi: spi-mtk-nor: use dma_alloc_coherent() for bounce buffer
From: Chuanhong Guo
Date: Fri Sep 18 2020 - 09:25:28 EST
Hi!
On Fri, Sep 18, 2020 at 4:35 PM Ikjoon Jang <ikjn@xxxxxxxxxxxx> wrote:
>
> Use dma_alloc_coherent() for bounce buffer instead of kmalloc.
The commit message should explain why such a change is
needed. (i.e. why using dma_alloc_coherent here is better
than kmalloc.) And if there's no benefit for this change I'd prefer
leaving it untouched.
I remembered reading somewhere that stream DMA api is
prefered over dma_alloc_coherent for this kind of single-direction
DMA operation.
>
> Signed-off-by: Ikjoon Jang <ikjn@xxxxxxxxxxxx>
>
> ---
>
> (no changes since v1)
>
> drivers/spi/spi-mtk-nor.c | 60 +++++++++++++++++++++++----------------
> 1 file changed, 35 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/spi/spi-mtk-nor.c b/drivers/spi/spi-mtk-nor.c
> index 54b2c0fde95b..e14798a6e7d0 100644
> --- a/drivers/spi/spi-mtk-nor.c
> +++ b/drivers/spi/spi-mtk-nor.c
> @@ -96,6 +96,7 @@ struct mtk_nor {
> struct device *dev;
> void __iomem *base;
> u8 *buffer;
> + dma_addr_t buffer_dma;
> struct clk *spi_clk;
> struct clk *ctlr_clk;
> unsigned int spi_freq;
> @@ -275,19 +276,16 @@ static void mtk_nor_setup_bus(struct mtk_nor *sp, const struct spi_mem_op *op)
> mtk_nor_rmw(sp, MTK_NOR_REG_BUSCFG, reg, MTK_NOR_BUS_MODE_MASK);
> }
>
> -static int mtk_nor_read_dma(struct mtk_nor *sp, u32 from, unsigned int length,
> - u8 *buffer)
> +static int read_dma(struct mtk_nor *sp, u32 from, unsigned int length,
This name is a bit confusing considering there's a mtk_nor_read_dma
below.
As this function now only executes dma readings and wait it to finish,
what about mtk_nor_dma_exec instead?
> + dma_addr_t dma_addr)
> {
> int ret = 0;
> ulong delay;
> u32 reg;
> - dma_addr_t dma_addr;
>
> - dma_addr = dma_map_single(sp->dev, buffer, length, DMA_FROM_DEVICE);
> - if (dma_mapping_error(sp->dev, dma_addr)) {
> - dev_err(sp->dev, "failed to map dma buffer.\n");
> + if (WARN_ON((length & MTK_NOR_DMA_ALIGN_MASK) ||
> + (dma_addr & MTK_NOR_DMA_ALIGN_MASK)))
These alignment is guaranteed by callers of this function if all
my comments below are addressed. This check isn't needed.
> return -EINVAL;
> - }
>
> writel(from, sp->base + MTK_NOR_REG_DMA_FADR);
> writel(dma_addr, sp->base + MTK_NOR_REG_DMA_DADR);
> @@ -312,30 +310,39 @@ static int mtk_nor_read_dma(struct mtk_nor *sp, u32 from, unsigned int length,
> (delay + 1) * 100);
> }
>
> - dma_unmap_single(sp->dev, dma_addr, length, DMA_FROM_DEVICE);
> if (ret < 0)
> dev_err(sp->dev, "dma read timeout.\n");
>
> return ret;
> }
>
> -static int mtk_nor_read_bounce(struct mtk_nor *sp, u32 from,
> - unsigned int length, u8 *buffer)
> +static int mtk_nor_read_dma(struct mtk_nor *sp, u32 from,
> + unsigned int length, u8 *buffer)
> {
> - unsigned int rdlen;
> int ret;
> + dma_addr_t dma_addr;
> + bool bounce = need_bounce(buffer, length);
>
> - if (length & MTK_NOR_DMA_ALIGN_MASK)
> - rdlen = (length + MTK_NOR_DMA_ALIGN) & ~MTK_NOR_DMA_ALIGN_MASK;
The intention of this rdlen alignment is explained in 2/5.
Please make sure this rdlen alignment logic is present
only for PIO reading.
> - else
> - rdlen = length;
> + if (!bounce) {
> + dma_addr = dma_map_single(sp->dev, buffer, length,
> + DMA_FROM_DEVICE);
> + if (dma_mapping_error(sp->dev, dma_addr)) {
> + dev_err(sp->dev, "failed to map dma buffer.\n");
> + return -EINVAL;
> + }
> + } else {
> + dma_addr = sp->buffer_dma;
> + }
>
> - ret = mtk_nor_read_dma(sp, from, rdlen, sp->buffer);
> - if (ret)
> - return ret;
> + ret = read_dma(sp, from, length, dma_addr);
>
> - memcpy(buffer, sp->buffer, length);
> - return 0;
> + if (!bounce)
> + dma_unmap_single(sp->dev, dma_addr, length,
> + DMA_FROM_DEVICE);
> + else
> + memcpy(buffer, sp->buffer, length);
> +
> + return ret;
> }
I think a separated read_dma and read_bounce function will be
cleaner than this if-else implementation:
read_dma:
1. call dma_map_single to get physical address
2. call read_dma to execute operation
3. call dma_unmap_single
read_bounce:
1. align reading length
2. call read_dma
3. call memcpy
>
> static int mtk_nor_read_pio(struct mtk_nor *sp, const struct spi_mem_op *op)
> @@ -439,11 +446,6 @@ static int mtk_nor_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
> if (op->data.nbytes == 1) {
> mtk_nor_set_addr(sp, op);
> return mtk_nor_read_pio(sp, op);
> - } else if (((ulong)(op->data.buf.in) &
> - MTK_NOR_DMA_ALIGN_MASK)) {
> - return mtk_nor_read_bounce(sp, op->addr.val,
> - op->data.nbytes,
> - op->data.buf.in);
> } else {
> return mtk_nor_read_dma(sp, op->addr.val,
> op->data.nbytes,
> @@ -654,6 +656,10 @@ static int mtk_nor_probe(struct platform_device *pdev)
> sp->dev = &pdev->dev;
> sp->spi_clk = spi_clk;
> sp->ctlr_clk = ctlr_clk;
There is extra memory allocation code for sp->buffer in mtk_nor_probe.
If you intend to replace this with dma_alloc_coherent you should
drop those devm_kmalloc code as well.
> + sp->buffer = dma_alloc_coherent(&pdev->dev, MTK_NOR_BOUNCE_BUF_SIZE,
> + &sp->buffer_dma, GFP_KERNEL);
There's a devm variant: dmam_alloc_coherent(dev, size, dma_handle, gfp)
> + if (!sp->buffer)
> + return -ENOMEM;
This spi-nor controller requires all addresses to be 16-byte aligned.
Although it should be guaranteed by a usually way larger page
alignment address from dma_alloc_coherent I'd prefer an explicit
check for address alignment here rather than letting it probe
successfully and fail for every dma_read with bounce buffer.
>
> irq = platform_get_irq_optional(pdev, 0);
> if (irq < 0) {
> @@ -674,6 +680,8 @@ static int mtk_nor_probe(struct platform_device *pdev)
> ret = mtk_nor_init(sp);
> if (ret < 0) {
> kfree(ctlr);
> + dma_free_coherent(&pdev->dev, MTK_NOR_BOUNCE_BUF_SIZE,
> + sp->buffer, sp->buffer_dma);
> return ret;
> }
>
> @@ -692,6 +700,8 @@ static int mtk_nor_remove(struct platform_device *pdev)
>
> mtk_nor_disable_clk(sp);
>
> + dma_free_coherent(&pdev->dev, MTK_NOR_BOUNCE_BUF_SIZE,
> + sp->buffer, sp->buffer_dma);
> return 0;
> }
>
> --
> 2.28.0.681.g6f77f65b4e-goog
>
--
Regards,
Chuanhong Guo