linux-kernel - Re: [PATCH v4 1/3] mtd: spi-nor: always use bounce buffer for register read/writes

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Message-ID: <b125bf29-f1fd-6d33-4a7c-49cb94ef1488@microchip.com>
Date:   Mon, 5 Aug 2019 09:06:47 +0000
From:   <Tudor.Ambarus@...rochip.com>
To:     <vigneshr@...com>, <miquel.raynal@...tlin.com>, <richard@....at>
CC:     <marek.vasut@...il.com>, <bbrezillon@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-mtd@...ts.infradead.org>,
        <tmaimon77@...il.com>
Subject: Re: [PATCH v4 1/3] mtd: spi-nor: always use bounce buffer for
 register read/writes



On 08/01/2019 07:22 PM, Vignesh Raghavendra wrote:
> External E-Mail
> 
> 
> spi-mem layer expects all buffers passed to it to be DMA'able. But
> spi-nor layer mostly allocates buffers on stack for reading/writing to
> registers and therefore are not DMA'able. Introduce bounce buffer to be
> used to read/write to registers. This ensures that buffer passed to
> spi-mem layer during register read/writes is DMA'able. With this change
> nor->cmd-buf is no longer used, so drop it.
> 
> Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
> Signed-off-by: Vignesh Raghavendra <vigneshr@...com>
> ---
> 
> v4:
> Avoid memcpy during READID
> 
> v3: new patch
> 
>  drivers/mtd/spi-nor/spi-nor.c | 70 ++++++++++++++++++++---------------
>  include/linux/mtd/spi-nor.h   |  7 +++-
>  2 files changed, 45 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
> index 03cc788511d5..e02376e1127b 100644
> --- a/drivers/mtd/spi-nor/spi-nor.c
> +++ b/drivers/mtd/spi-nor/spi-nor.c

cut

>  /**
> @@ -1404,9 +1401,11 @@ static int write_sr_cr(struct spi_nor *nor, u8 *sr_cr)
>  {
>  	int ret;
>  
> +	memcpy(nor->bouncebuf, sr_cr, 2);

I'm thinking out loud. This can be avoided by forcing all the callers to use
nor->bouncebuf. That would result in a:

static int write_sr(struct spi_nor *nor, size_t len)

write_sr_cr() can be removed. Memcopying 2 bytes is a small price to pay, we can
keep things as they are, to not be too invasive. But if you think that this idea
is worth it, tell.

> +
>  	write_enable(nor);
>  
> -	ret = nor->write_reg(nor, SPINOR_OP_WRSR, sr_cr, 2);
> +	ret = nor->write_reg(nor, SPINOR_OP_WRSR, nor->bouncebuf, 2);
>  	if (ret < 0) {
>  		dev_err(nor->dev,
>  			"error while writing configuration register\n");

cut

> @@ -2177,9 +2176,10 @@ static const struct flash_info spi_nor_ids[] = {
>  static const struct flash_info *spi_nor_read_id(struct spi_nor *nor)
>  {
>  	int			tmp;
> -	u8			id[SPI_NOR_MAX_ID_LEN];
> +	u8			*id;
>  	const struct flash_info	*info;
>  
> +	id = nor->bouncebuf;

nit: do init at declaration.

Also, you missed a place in which you can use the bouncebuf, search by "read_reg(":
ret = nor->read_reg(nor, SPINOR_OP_XRDSR, &val, 1);

Cheers,
ta