Re: [PATCH V4 1/5] mmc: Add erase, secure erase, trim and securetrim operations

[Adrian Hunter]

Ben Gardiner wrote:
> On Wed, Jul 7, 2010 at 7:17 AM, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> > From 7f01ad3c4be6ec09318176db12db66f353b526e0 Mon Sep 17 00:00:00 2001
>
> > SD/MMC cards tend to support an erase operation.  In addition,
> > eMMC v4.4 cards can support secure erase, trim and secure trim
> > operations that are all variants of the basic erase command.
>
> This is great. I am interested primarily in SD media.
>
> Please forgive my naive perspective: it seems that with the features
> enabled by this patchset and a filesystem that is capable of issuing
> erase block commands, the wear-leveling on SD media will be improved
> -- much like with CF TRIM commands. Do you also think that is the
> case? I would be very interested in hearing your expert opinion on
> this.

I am sorry but I don't know.  Wear-levelling in cards tends to be kept
secret by the manufacturers.  However, it is not clear to me that cards
bother to record whether or not anything has been erased.   For example,
erase a card twice - it takes the same amount of time the second time
as the first time, whereas if the card knew it was already erased, why
wasn't the second time much quicker?

> I have a couple comments regarding mostly the SD support introduced in
> this patch. Patches 2..5 of 5 seem fine to me but I'm not sure I'm
> qualified to add acks or reviewed-by's.
>
> > +int mmc_erase(struct mmc_card *card, unsigned int from, unsigned int nr,
> > +             unsigned int arg)
> > +{
> > +       unsigned int rem, to = from + nr;
> > +
> > +       if (!(card->host->caps & MMC_CAP_ERASE) ||
> > +           !(card->csd.cmdclass & CCC_ERASE))
> > +               return -EOPNOTSUPP;
> > +
> > +       if (!card->erase_size)
> > +               return -EOPNOTSUPP;
> > +
> > +       if (mmc_card_sd(card) && arg != MMC_ERASE_ARG)
> > +               return -EOPNOTSUPP;
> > +
> > +       if ((arg & MMC_SECURE_ARGS) &&
> > +           !(card->ext_csd.sec_feature_support & EXT_CSD_SEC_ER_EN))
> > +               return -EOPNOTSUPP;
> > +
> > +       if ((arg & MMC_TRIM_ARGS) &&
> > +           !(card->ext_csd.sec_feature_support & EXT_CSD_SEC_GB_CL_EN))
> > +               return -EOPNOTSUPP;
> > +
>
> > +int mmc_can_trim(struct mmc_card *card)
> > +{
> > +       if (card->ext_csd.sec_feature_support & EXT_CSD_SEC_GB_CL_EN)
> > +               return 1;
> > +       return 0;
> > +}
> > +EXPORT_SYMBOL(mmc_can_trim);
>
> It looks like mmc_can_trim(card) would return true when
> mmc_card_sd(card) is true;

It will return false for SD.  card->ext_csd.sec_feature_support
is only used by MMC.

> but the mmc_erase function will return
> -EOPNOTSUPP in such a case. I think that this results in the
> mmc_blk_issue_discard_rq function (from 2/5) also returning
> -EOPNOTSUPP whenever mmc_card_sd(card) is true:
>
> > From 2/5:
> +       if (mmc_can_trim(card))
> +               arg = MMC_TRIM_ARG;
> +       else
> +               arg = MMC_ERASE_ARG;
> +
> +       err = mmc_erase(card, from, nr, arg);
>
> Also, there is some duplication between the sec_feature_support
> checking in mmc_erase and in the mmc_can* functions; If mmc_erase
> could call the mmc_can_* functions then the the bit-checking logic
> could be centralized.

I can do that if there is a V5 but I do not think it is worth
rolling another set of patches just for that.

> > /*
> > + * Fetch and process SD Status register.
> > + */
> > +static int mmc_read_ssr(struct mmc_card *card)
> > +{
>
> It looks like the conventional function prefix for SD-specific
> functions in the rest of this file is mmc_sd_ ; 'mmc_read_ssr' ->
> 'mmc_sd_read_ssr'  or -> 'mmc_read_sd_sr' perhaps?

Well there is also mmc_decode_*, and mmc_read_switch so the other
functions that do smilar things also do not follow that convention.

> > +       ssr = kmalloc(64, GFP_KERNEL);
>
> Why '64' instead of 'sizeof(*ssr)' ?

sizeof(*ssr) is 4

> Best Regards,
> Ben Gardiner
>
> ---
> Nanometrics Inc.
> http://www.nanometrics.ca

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