linux-ext4 - discard feature, mkfs.ext4 and mmc default fallback to normal erase op

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

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