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

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Message-ID: <20151119231857.GM64635@google.com>
Date:	Thu, 19 Nov 2015 15:18:57 -0800
From:	Brian Norris <computersforpeace@...il.com>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Hou Zhiqiang <B48286@...escale.com>, shijie.huang@...el.com,
	David Woodhouse <dwmw2@...radead.org>,
	Han Xu <han.xu@...escale.com>,
	Rafał Miłecki <zajec5@...il.com>,
	Huang Shijie <b32955@...escale.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Marek Vasut <marex@...x.de>, Gabor Juhos <juhosg@...nwrt.org>,
	Bean Huo 霍斌斌, <beanhuo@...ron.com>,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/7] mtd: spi-nor: simplify write loop

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@...il.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
> 
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