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Date:   Fri, 07 May 2021 20:14:34 +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: <20210507180424.kj7c4rfjbycjagxm@ti.com>
References: <20210506191829.8271-1-p.yadav@ti.com>
 <20210506191829.8271-6-p.yadav@ti.com>
 <3daadf43ef4743f13ebbdd000ba5ec4a@walle.cc>
 <20210507180424.kj7c4rfjbycjagxm@ti.com>
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Am 2021-05-07 20:04, schrieb Pratyush Yadav:
> 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.

Ah no, I wasn't talking about your implementation, but just having a 
naive
one where you don't move around up to PAGE_SIZE of data but just read
2 bytes in the beginning (if unaligned) and 2 bytes at the end (if 
unaligned)
and reading the part in between just as usual because its then aligend.

> 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.

Yes, with this you will have that memcpy() and one transfer for 
transfers
up to PAGE_SIZE; the "naive" one above would have up to three depending 
on
the aligment.

> 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.

-michael