Re: [PATCH v4 6/7] mtd: spi-nor: simplify write loop

Brian Norris
2015-11-19T23:19:03Z

Hi Michal,

Sorry this has sat so long...

On Fri, Aug 14, 2015 at 09:23:08AM -0000, Michal Suchanek wrote:
> The spi-nor write loop assumes that what is passed to the hardware
> driver write() is what gets written.
>
> When write() writes less than page size at once data is dropped on the
> floor. Check the amount of data writen.

Have you seen write() return less than the page size? I know you've
struggled with a SPI driver that can't do "very" (for some definition of
very) long transfers, due to unknown bugs, but I thought that "very" was
much larger than the page size.

> This also means that write can start mid-page any time so there is
> no special case for first page.

I think we'd have some problems if we start seeing hardware that can't
write ~256 bytes at a time. If nothing else, this can be a problem
because some SPI NOR flash are known to have inferior reliability if you
regularly write in small chunks. I believe this is because they actually
use some kind of internal ECC.

So, if you're just guarding against a theoretical behavior, perhaps it's
best if this is done with some kind of assertion, as I'd rather not
encourage non-aligned writes if possible. I notice you used BUG_ON() in
another patch, but I'd suggest maybe something less harsh, like WARN()
or WARN_ONCE().

Brian

>
> Signed-off-by: Michal Suchanek <hramrach@gmail.com>
> ---
> drivers/mtd/spi-nor/spi-nor.c | 48 ++++++++++++++-----------------------------
> 1 file changed, 15 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
> index 837e3df..e0ae9cf 100644
> --- a/drivers/mtd/spi-nor/spi-nor.c
> +++ b/drivers/mtd/spi-nor/spi-nor.c
> @@ -836,7 +836,7 @@ static int spi_nor_write(struct mtd_info *mtd, loff_t to, size_t len,
> size_t *retlen, const u_char *buf)
> {
> struct spi_nor *nor = mtd_to_spi_nor(mtd);
> - u32 page_offset, page_size, i;
> + size_t page_offset, page_remain, i;
> int ret;
>
> dev_dbg(nor->dev, "to 0x%08x, len %zd\n", (u32)to, len);
> @@ -845,45 +845,27 @@ static int spi_nor_write(struct mtd_info *mtd, loff_t to, size_t len,
> if (ret)
> return ret;
>
> - write_enable(nor);
> -
> - page_offset = to & (nor->page_size - 1);
> + for (i = 0; i < len; ) {
> + int written;
>
> - /* do all the bytes fit onto one page? */
> - if (page_offset + len <= nor->page_size) {
> - ret = nor->write(nor, to, len, buf);
> - if (ret < 0)
> - goto write_err;
> - *retlen += ret;
> - } else {
> + page_offset = to & (nor->page_size - 1);
> /* the size of data remaining on the first page */
> - page_size = nor->page_size - page_offset;
> - ret = nor->write(nor, to, page_size, buf);
> + page_remain = min_t(size_t,
> + nor->page_size - page_offset, len - i);
> +
> + write_enable(nor);
> + ret = nor->write(nor, to + i, page_remain, buf + i);
> if (ret < 0)
> goto write_err;
> - *retlen += ret;
> -
> - /* write everything in nor->page_size chunks */
> - for (i = ret; i < len; ) {
> - page_size = len - i;
> - if (page_size > nor->page_size)
> - page_size = nor->page_size;
> -
> - ret = spi_nor_wait_till_ready(nor);
> - if (ret)
> - goto write_err;
> + written = ret;
>
> - write_enable(nor);
> -
> - ret = nor->write(nor, to + i, page_size, buf + i);
> - if (ret < 0)
> - goto write_err;
> - *retlen += ret;
> - i += ret;
> - }
> + ret = spi_nor_wait_till_ready(nor);
> + if (ret)
> + goto write_err;
> + *retlen += written;
> + i += written;
> }
>
> - ret = spi_nor_wait_till_ready(nor);
> write_err:
> spi_nor_unlock_and_unprep(nor, SPI_NOR_OPS_WRITE);
> return ret;
> --
> 2.1.4
>