Re: [PATCH 5/6] mtd: spi-nor: core; avoid odd length/address reads on 8D-8D-8D mode

Michael Walle
2021-05-07T15:51:49Z

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