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Date:   Fri, 07 May 2021 17:51:07 +0200
From:   Michael Walle <michael@walle.cc>
To:     Pratyush Yadav <p.yadav@ti.com>
Cc:     Tudor Ambarus <tudor.ambarus@microchip.com>,
        Miquel Raynal <miquel.raynal@bootlin.com>,
        Richard Weinberger <richard@nod.at>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Mark Brown <broonie@kernel.org>, linux-mtd@lists.infradead.org,
        linux-kernel@vger.kernel.org, linux-spi@vger.kernel.org
Subject: Re: [PATCH 5/6] mtd: spi-nor: core; avoid odd length/address reads on
 8D-8D-8D mode
In-Reply-To: <20210506191829.8271-6-p.yadav@ti.com>
References: <20210506191829.8271-1-p.yadav@ti.com>
 <20210506191829.8271-6-p.yadav@ti.com>
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Am 2021-05-06 21:18, schrieb Pratyush Yadav:
> On Octal DTR capable flashes like Micron Xcella reads cannot start or
> end at an odd address in Octal DTR mode. Extra bytes need to be read at
> the start or end to make sure both the start address and length remain
> even.
> 
> To avoid allocating too much extra memory, thereby putting unnecessary
> memory pressure on the system, the temporary buffer containing the 
> extra
> padding bytes is capped at PAGE_SIZE bytes. The rest of the 2-byte
> aligned part should be read directly in the main buffer.
> 
> Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
> ---
> 
>  drivers/mtd/spi-nor/core.c | 81 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 80 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
> index 5cc206b8bbf3..3d66cc34af4d 100644
> --- a/drivers/mtd/spi-nor/core.c
> +++ b/drivers/mtd/spi-nor/core.c
> @@ -1904,6 +1904,82 @@ static const struct flash_info
> *spi_nor_read_id(struct spi_nor *nor)
>  	return ERR_PTR(-ENODEV);
>  }
> 
> +/*
> + * On Octal DTR capable flashes like Micron Xcella reads cannot start 
> or
> + * end at an odd address in Octal DTR mode. Extra bytes need to be 
> read
> + * at the start or end to make sure both the start address and length
> + * remain even.
> + */
> +static int spi_nor_octal_dtr_read(struct spi_nor *nor, loff_t from, 
> size_t len,
> +				  u_char *buf)
> +{
> +	u_char *tmp_buf;
> +	size_t tmp_len;
> +	loff_t start, end;
> +	int ret, bytes_read;
> +
> +	if (IS_ALIGNED(from, 2) && IS_ALIGNED(len, 2))
> +		return spi_nor_read_data(nor, from, len, buf);
> +	else if (IS_ALIGNED(from, 2) && len > PAGE_SIZE)
> +		return spi_nor_read_data(nor, from, round_down(len, PAGE_SIZE),
> +					 buf);
> +
> +	tmp_buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
> +	if (!tmp_buf)
> +		return -ENOMEM;
> +
> +	start = round_down(from, 2);
> +	end = round_up(from + len, 2);
> +
> +	/*
> +	 * Avoid allocating too much memory. The requested read length might 
> be
> +	 * quite large. Allocating a buffer just as large (slightly bigger, 
> in
> +	 * fact) would put unnecessary memory pressure on the system.
> +	 *
> +	 * For example if the read is from 3 to 1M, then this will read from 
> 2
> +	 * to 4098. The reads from 4098 to 1M will then not need a temporary
> +	 * buffer so they can proceed as normal.
> +	 */
> +	tmp_len = min_t(size_t, end - start, PAGE_SIZE);
> +
> +	ret = spi_nor_read_data(nor, start, tmp_len, tmp_buf);
> +	if (ret == 0) {
> +		ret = -EIO;
> +		goto out;
> +	}
> +	if (ret < 0)
> +		goto out;
> +
> +	/*
> +	 * More bytes are read than actually requested, but that number can't 
> be
> +	 * reported to the calling function or it will confuse its 
> calculations.
> +	 * Calculate how many of the _requested_ bytes were read.
> +	 */
> +	bytes_read = ret;
> +
> +	if (from != start)
> +		ret -= from - start;
> +
> +	/*
> +	 * Only account for extra bytes at the end if they were actually 
> read.
> +	 * For example, if the total length was truncated because of 
> temporary
> +	 * buffer size limit then the adjustment for the extra bytes at the 
> end
> +	 * is not needed.
> +	 */
> +	if (start + bytes_read == end)
> +		ret -= end - (from + len);
> +
> +	if (ret < 0) {
> +		ret = -EIO;
> +		goto out;
> +	}
> +
> +	memcpy(buf, tmp_buf + (from - start), ret);
> +out:
> +	kfree(tmp_buf);
> +	return ret;
> +}
> +
>  static int spi_nor_read(struct mtd_info *mtd, loff_t from, size_t len,
>  			size_t *retlen, u_char *buf)
>  {
> @@ -1921,7 +1997,10 @@ static int spi_nor_read(struct mtd_info *mtd,
> loff_t from, size_t len,
> 
>  		addr = spi_nor_convert_addr(nor, addr);
> 
> -		ret = spi_nor_read_data(nor, addr, len, buf);
> +		if (nor->read_proto == SNOR_PROTO_8_8_8_DTR)
> +			ret = spi_nor_octal_dtr_read(nor, addr, len, buf);
> +		else
> +			ret = spi_nor_read_data(nor, addr, len, buf);
>  		if (ret == 0) {
>  			/* We shouldn't see 0-length reads */
>  			ret = -EIO;

Reviewed-by: Michael Walle <michael@walle.cc>

I wonder how much performance is lost if this would just split
one transfer into up to three ones: 2 byte, size - 2, 2 bytes.

-michael