linux-kernel - Re: [RFC PATCH 2/2] mmc: core.c: Add comment to clarify special cases of ERASE/TRIM

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Date:	Thu, 04 Jun 2015 14:16:23 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	David Jander <david@...tonic.nl>
CC:	Ulf Hansson <ulf.hansson@...aro.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Johan Rudholm <johan.rudholm@...s.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] mmc: core.c: Add comment to clarify special cases
 of ERASE/TRIM

On 04/06/15 13:20, David Jander wrote:
> Signed-off-by: David Jander <david@...tonic.nl>

Please never send delta patches.  Always send a new version of the whole patch.

> ---
>  drivers/mmc/core/core.c | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index 6c9611b..b6aa9ad 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -2109,11 +2109,20 @@ int mmc_erase(struct mmc_card *card, unsigned int from, unsigned int nr,
>  	    !(card->ext_csd.sec_feature_support & EXT_CSD_SEC_GB_CL_EN))
>  		return -EOPNOTSUPP;
>  
> +	/*
> +	 * Sanity check: If we do not erase aligned, whole erase-groups, return
> +	 * an error, since we intended a "secure" erase, silently not erasing
> +	 * something would be unacceptable.
> +	 */

I am not sure the value of a comment that can anyway be inferred from the code.

>  	if (arg == MMC_SECURE_ERASE_ARG) {
>  		if (from % card->erase_size || nr % card->erase_size)
>  			return -EINVAL;
>  	}
>  
> +	/*
> +	 * Make sure only erase-groups that are fully contained in the erase
> +	 * region are erased. Silently ignore the rest.
> +	 */

Ditto

>  	if (arg == MMC_ERASE_ARG) {
>  		rem = from % card->erase_size;
>  		if (rem) {
> @@ -2140,6 +2149,14 @@ int mmc_erase(struct mmc_card *card, unsigned int from, unsigned int nr,
>  	/* 'from' and 'to' are inclusive */
>  	to -= 1;
>  
> +	/*
> +	 * Special case where only one erase-group fits in the timout budget:

timout -> timeout

> +	 * If the region crosses an erase-group boundary on this particular
> +	 * case, we will be trimming more than one erase-group which, does not
> +	 * fit in the timeout budget of the controller, so we need to split it
> +	 * and call mmc_do_erase() twice if necessary. This special case is
> +	 * identified by the card->eg_boundary flag.
> +	 */
>  	if ((arg & MMC_TRIM_ARGS) && (card->eg_boundary) &&
>  	    (from % card->erase_size)) {
>  		rem = card->erase_size - (from % card->erase_size);
> @@ -2244,7 +2261,16 @@ static unsigned int mmc_do_calc_max_discard(struct mmc_card *card,
>  	if (!qty)
>  		return 0;
>  
> -	/* We can only erase one erase group special case */
> +	/*
> +	 * When specifying a sector range to trim, chances are we might cross
> +	 * an erase-group boundary even if the amount of sectors is less than
> +	 * one erase-group.
> +	 * If we can only fit one erase-group in the controller timeout budget,
> +	 * we have to care that erase-group boundaries are not crossed by a
> +	 * single trim operation. We flag that special case with "eg_boundary".
> +	 * In all other cases we can just decrement qty and pretend that we
> +	 * always touch (qty + 1) erase-groups as a simple optimization.

The language seems a little odd here. We are setting the max_discard limit
which does not involve "pretending" or "optimization", it is just a
calculation.  The important point is that the calculation has to count the
maximum number of erase blocks affected not the size in erase blocks.  You
could give an example e.g. if a 2 sector trim crosses an erase block
boundary then that counts as 2 erase blocks affected.

> +	 */
>  	if (qty == 1)
>  		card->eg_boundary = 1;
>  	else
> 

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