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

[Pratyush Yadav <p.yadav@ti.com>]

On 07/05/21 05:51PM, Michael Walle wrote:
> 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>

Thanks.

> 
> 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.

This case is not really possible since it would try to read PAGE_SIZE 
whenever it can. But there is a situation possible where one transfer is 
split into three. It would look something like: 4096 bytes, size - 4096 
bytes, 2 bytes.

I am trying to find a balance between minimizing number of reads while 
keeping the size of the temporary buffer to a reasonable limit. This is 
the best I could come up with. It optimizes for smaller transfers so 
while the absolute amount of overhead remains roughly the same, the 
ratio of it relative to read size is smaller.

You can optimize for read performance if you are willing to waste memory 
by simple allocating a size + 2 bytes long buffer. Then the read can 
proceed in one transaction. But IMO memory is much more important 
compared to read throughput.

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.