Re: [PATCH] mmc: core: don't return 1 for max_discard

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On Thu, Dec 19, 2013 at 6:27 AM, Stephen Warren <swarren@xxxxxxxxxxxxx> wrote:
> From: Stephen Warren <swarren@xxxxxxxxxx>
>
> In mmc_do_calc_max_discard(), if only a single erase block can be
> discarded within the host controller's timeout, don't allow discard
> operations at all.
>
> Previously, the code allowed sector-at-a-time discard (rather than
> erase-block-at-a-time), which was chronically slow.
>
> Without this patch, on the NVIDIA Tegra Cardhu board, the loops result
> in qty == 1, which is immediately returned. This causes discard to
> operate a single sector at a time, which is chronically slow. With this
> patch in place, discard operates a single erase block at a time, which
> is reasonably fast.
>
> Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Cc: Dong Aisheng <dongas86@xxxxxxxxx>
> Cc: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> Cc: Vladimir Zapolskiy <vz@xxxxxxxxx>
> Fixes: e056a1b5b67b "(mmc: queue: let host controllers specify maximum discard timeout")
> Signed-off-by: Stephen Warren <swarren@xxxxxxxxxx>
> ---
>  drivers/mmc/core/core.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index 57a2b403bf8e..eb952ca634ea 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -2150,8 +2150,25 @@ static unsigned int mmc_do_calc_max_discard(struct mmc_card *card,
>         if (!qty)
>                 return 0;
>
> -       if (qty == 1)
> -               return 1;
> +       /*
> +        * Discard operations may not be aligned to an erase block. In an
> +        * unaligned case, we need to issue 1 more discard operation to HW
> +        * than strictly calculated by:
> +        *     sectors_to_erase / sectors_per_discard_operation
> +        *
> +        * To account for this in the timeout calculations, we assume we can
> +        * actually discard one less erase block than fits into the HW
> +        * timeout. This explains the --qty below.
> +        *
> +        * When only a single discard block operation fits into the timeout,
> +        * disallow any discard operations at all. For example, discarding one
> +        * sector at a time is so chronically slow as to be useless. However,
> +        * make an exception for SD cards without an erase shift, since qty
> +        * isn't multiplied up by an erase block size in the code below for
> +        * that case.
> +        */
> +       if (qty == 1 && !(!card->erase_shift && mmc_card_sd(card)))
> +               return 0;
>

How about when qty == 2?
Erase 2 sectors may have no much difference from 1 sector per one time
for a SD card,
it may still chronically slow, i guess.
So i wonder it may not sovle the issues totally.

Regards
Dong Aisheng

>         /* Convert qty to sectors */
>         if (card->erase_shift)
> --
> 1.8.1.5
>
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