Re: max_discard anomaly on certain Sandisk eMMC

Adrian Hunter
2013-12-17T11:20:19Z

On 17/12/13 12:04, Ulf Hansson wrote:
> On 17 December 2013 09:17, Adrian Hunter <adrian.hunter-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> wrote:
>> 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.
>
> Don't know the details about the limitations for SDHCI, but I guess
> similar exists for other controllers as well.

Not necessarily. For example omap_hsmmc just disables the timeout for erase
operations.

>
> I do get the impression that we have got a problem in the mmc
> core/block layer for how erase/trim/discard timeouts are being
> handled.
>
> I don't think the mmc hw-controller should be waiting for the R1B
> response from the CMD38 as long as this "timeout" we are discussing
> here. According to the spec, at least how I interpret it, the card
> should respond rather quickly to CMD38, then it will assert the DAT0
> line to indicate busy.
>
> The total time the card is allowed to stay busy, that is what the
> timeout specifies. We may either use a mmc hw-controller busy
> detection mechanism or send CMD13 to poll for status. The latter is
> somewhat already being handled in mmc_do_erase(), but we are using
> "MMC_CORE_TIMEOUT_MS" instead of the correct timeout.
>
> Kind regards
> Ulf Hansson
>
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-mmc" in
>> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>
>