Re: max_discard anomaly on certain Sandisk eMMC

[Adrian Hunter <adrian.hunter@intel.com>]

On 17/12/13 01:18, Stephen Warren wrote:
> On 12/13/2013 03:43 PM, Stephen Warren wrote:
>> On one of my eMMC devices, I see the following results from calling
>> mmc_do_calc_max_discard() with various parameters:
>>
>> [    3.057263] MMC_DISCARD_ARG max_discard 1
>> [    3.057266] MMC_ERASE_ARG   max_discard 4096
>> [    3.057267] MMC_TRIM_ARG    max_discard 1
>>
>> This causes mmc_calc_max_discard() to return 1, which makes the discard
>> IOCTL extremely slow.
> 
> Further investigation shows that if I make a few hacks that essentially
> revert e056a1b5b67b "mmc: queue: let host controllers specify maximum
> discard timeout":
> 
> diff --git a/drivers/mmc/card/queue.c b/drivers/mmc/card/queue.c
> index 357bbc54fe4b..e66af930d0e3 100644
> --- a/drivers/mmc/card/queue.c
> +++ b/drivers/mmc/card/queue.c
> @@ -167,13 +167,15 @@ static void mmc_queue_setup_discard(struct
> request_queue *q,
>  		return;
> 
>  	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
> -	q->limits.max_discard_sectors = max_discard;
> +	q->limits.max_discard_sectors = UINT_MAX;
>  	if (card->erased_byte == 0 && !mmc_can_discard(card))
>  		q->limits.discard_zeroes_data = 1;
>  	q->limits.discard_granularity = card->pref_erase << 9;
>  	/* granularity must not be greater than max. discard */
> +#if 0
>  	if (card->pref_erase > max_discard)
>  		q->limits.discard_granularity = 0;
> +#endif
>  	if (mmc_can_secure_erase_trim(card))
>  		queue_flag_set_unlocked(QUEUE_FLAG_SECDISCARD, q);
>  }
> 
> I end up with:
> 
> $ cat /sys/.../block/mmcblk1/queue# cat discard_granularity
> 2097152
> $ cat /sys/.../block/mmcblk1/queue# cat discard_max_bytes
> 2199023255040
> $ cat /sys/.../block/mmcblk1/queue# cat discard_zeroes_data
> 1
> 
> With those values, mke2fs is fast, and I validated that "blkdiscard"
> works; I filled a large partition with /dev/urandom, executed
> "blkdiscard" on the 4M at the start, and saw zeroes when reading the
> discarded part back.
> 
> This implies that the issue is simply the operation of
> mmc_calc_max_discard(), rather than the eMMC device mis-reporting its
> discard abilities, doesn't it?

No.

The underlying problem is a combination of:
	a) JEDEC specified very large timeouts for erase operations e.g. can be
minutes for large erases
	b) SDHCI controllers have been implemented with high frequency timeout
clocks which limit the maximum timeout to a few seconds
	c) It is not possible to disable the timeout on SDHCI

What a) means is that you can get away with much larger erases than you can
specify the timeout for - which is what you have discovered.

To understand the timeouts, you should manually do the calculations.

Also note, that using HC Erase Size may help (MMC_CAP2_HC_ERASE_SZ), but
beware of the partitioning implications of changing that.

The best solution is to change the hardware to use the lowest possible
frequency timeout clock e.g. a 1KHz timeout clock could support timeouts of
up to 36 hours.