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Date:   Mon, 07 Dec 2020 16:10:27 +0100
From:   Michael Walle <michael@walle.cc>
To:     linux-ext4@vger.kernel.org, linux-mmc@vger.kernel.org,
        linux-block@vger.kernel.org
Subject: discard feature, mkfs.ext4 and mmc default fallback to normal erase
 op
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Hi,

The problem I'm having is that I'm trying to install debian on
an embedded system onto an sdcard. During installation it will
format the target filesystem, but the "mkfs.ext4 -F /dev/mmcblk0p2"
takes ages.

What I've found out so far:
  - mkfs.ext4 tries to discard all blocks on the target device
  - with my target device being an sdcard it seems to fallback
    to normal erase [1], with erase_arg being set to what the card
    is capable of [2]

Now I'm trying to figure out if this behavior is intended. I guess
one can reduce it to "blkdiscard /dev/mmcblk0p2". Should this
actually fall back to normal erasing or should it return -EOPNOTSUPP?

-michael

[1] 
https://elixir.bootlin.com/linux/v5.9.12/source/drivers/mmc/core/block.c#L1063
[2] 
https://elixir.bootlin.com/linux/v5.9.12/source/drivers/mmc/core/mmc.c#L1751