Re: [PATCH v2 6/6] mtd: spi-nor: core: avoid odd length/address writes in 8D-8D-8D mode

[<Tudor.Ambarus@xxxxxxxxxxxxx>]

On 5/31/21 9:17 PM, Pratyush Yadav wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On Octal DTR capable flashes like Micron Xcella the writes cannot start
> or end at an odd address in Octal DTR mode. Extra 0xff bytes need to be
> appended or prepended to make sure the start address and end address are
> even. 0xff is used because on NOR flashes a program operation can only
> flip bits from 1 to 0, not the other way round. 0 to 1 flip needs to
> happen via erases.
> 
> Signed-off-by: Pratyush Yadav <p.yadav@xxxxxx>
> 
> ---
> 
> Changes in v2:
> - Replace semicolon in subject with colon.
> - Add a comment that ret < 0 after adjusting for extra bytes is not
>   possible, and add a WARN_ON() on the condition to make sure it gets
>   spotted quickly when some change triggers this bug.
> 
>  drivers/mtd/spi-nor/core.c | 73 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 72 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
> index a696af6a1b71..d2a7e16e667d 100644
> --- a/drivers/mtd/spi-nor/core.c
> +++ b/drivers/mtd/spi-nor/core.c
> @@ -2023,6 +2023,72 @@ static int spi_nor_read(struct mtd_info *mtd, loff_t from, size_t len,
>         return ret;
>  }
> 
> +/*

let's add kernel-doc comments for new methods.

> + * On Octal DTR capable flashes like Micron Xcella the writes cannot start or

strip "the" from "the writes". But I think I would get rid of the Micron Xcella example,
we're using these methods for all the 8D-8D-8D flashes. You can mention Micron in the
commit message if you want, but we shouldn't mention manufacturers in the core.

> + * end at an odd address in Octal DTR mode. Extra 0xff bytes need to be appended
> + * or prepended to make sure the start address and end address are even. 0xff is
> + * used because on NOR flashes a program operation can only flip bits from 1 to
> + * 0, not the other way round. 0 to 1 flip needs to happen via erases.
> + */
> +static int spi_nor_octal_dtr_write(struct spi_nor *nor, loff_t to, size_t len,
> +                                  const u8 *buf)
> +{
> +       u8 *tmp_buf;
> +       size_t bytes_written;
> +       loff_t start, end;
> +       int ret;
> +
> +       if (IS_ALIGNED(to, 2) && IS_ALIGNED(len, 2))
> +               return spi_nor_write_data(nor, to, len, buf);
> +
> +       tmp_buf = kmalloc(nor->page_size, GFP_KERNEL);> +       if (!tmp_buf)
> +               return -ENOMEM;
> +
> +       memset(tmp_buf, 0xff, nor->page_size);
> +
> +       start = round_down(to, 2);
> +       end = round_up(to + len, 2);
> +
> +       memcpy(tmp_buf + (to - start), buf, len);
> +
> +       ret = spi_nor_write_data(nor, start, end - start, tmp_buf);
> +       if (ret == 0) {
> +               ret = -EIO;
> +               goto out;
> +       }
> +       if (ret < 0)
> +               goto out;
> +
> +       /*
> +        * More bytes are written 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
> +        * written.
> +        */
> +       bytes_written = ret;
> +
> +       if (to != start)
> +               ret -= to - start;
> +
> +       /*
> +        * Only account for extra bytes at the end if they were actually
> +        * written. For example, if for some reason the controller could only
> +        * complete a partial write then the adjustment for the extra bytes at
> +        * the end is not needed.
> +        */
> +       if (start + bytes_written == end)
> +               ret -= end - (to + len);
> +
> +       /* Should not be possible. */
> +       if (WARN_ON(ret < 0))
> +               ret = -EIO;

Is this really needed? Also, I think I would squash patch 5 and 6,
it's the same idea, and reads and writes are tied together.

Looks good!

> +
> +out:
> +       kfree(tmp_buf);
> +       return ret;
> +}
> +
>  /*
>   * Write an address range to the nor chip.  Data must be written in
>   * FLASH_PAGESIZE chunks.  The address range may be any size provided
> @@ -2067,7 +2133,12 @@ static int spi_nor_write(struct mtd_info *mtd, loff_t to, size_t len,
>                 if (ret)
>                         goto write_err;
> 
> -               ret = spi_nor_write_data(nor, addr, page_remain, buf + i);
> +               if (nor->write_proto == SNOR_PROTO_8_8_8_DTR)
> +                       ret = spi_nor_octal_dtr_write(nor, addr, page_remain,
> +                                                     buf + i);
> +               else
> +                       ret = spi_nor_write_data(nor, addr, page_remain,
> +                                                buf + i);
>                 if (ret < 0)
>                         goto write_err;
>                 written = ret;
> --
> 2.30.0
>