On 12/17/13 11:04, Ulf Hansson wrote:
On 17 December 2013 09:17, Adrian Hunter<adrian.hunter@xxxxxxxxx> 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.
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.
What is the correct timeout? The currently implemented logic doesn't
allow to set let say 1000 erase groups (and the correspondent timeout),
on the other hand if it is allowed, then this correct timeout may
take tens of hours, which should not be permitted from user's
perspective. I think the predefined MMC_CORE_TIMEOUT_MS is good enough,
the only missing part is a permission to erase as many erase groups
as wanted by user.
With best wishes,
Vladimir
Kind regards
Ulf Hansson
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