Re: [PATCH] mmc: core: Fix off-by-one error in mmc_do_calc_max_discard()

Adrian Hunter
2015-06-01T10:39:07Z

On 01/06/15 12:20, David Jander wrote:
> qty is the maximum number of discard that _do_ fit in the timeout, not
> the first amount that does _not_ fit anymore.
> This seemingly harmless error has a very severe performance impact when
> the timeout value is enough for only 1 erase group.
>
> Signed-off-by: David Jander <david@protonic.nl>
> ---
> drivers/mmc/core/core.c | 7 ++-----
> 1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index 92e7671..1f9573b 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -2234,16 +2234,13 @@ static unsigned int mmc_do_calc_max_discard(struct mmc_card *card,
> if (!qty)
> return 0;
>
> - if (qty == 1)
> - return 1;
> -
> /* Convert qty to sectors */
> if (card->erase_shift)
> - max_discard = --qty << card->erase_shift;
> + max_discard = qty << card->erase_shift;
> else if (mmc_card_sd(card))
> max_discard = qty;
> else
> - max_discard = --qty * card->erase_size;
> + max_discard = qty * card->erase_size;
>
> return max_discard;
> }
>

This keeps coming up but there is more to it than that. See here: