[PATCH] mtd: spi-nor: Prefer asynchronous probe

[Michael Walle]

Hi Douglas,

> On my system the spi_nor_probe() took ~6 ms at bootup.  That's not a
> lot, but every little bit adds up to a slow bootup.  While we can get
> this out of the boot path by making it a module, there are times where
> it is convenient (or even required) for this to be builtin the kernel.
> Let's set that we prefer async probe so that we don't block other
> drivers from probing while we are probing.
> 
> This is a tiny little change that is almost guaranteed to be safe for
> anything that is able to run as a module, which SPI_NOR is.
> Specifically modules are already probed asynchronously.  Also: since
> other things in the system may have enabled asynchronous probe the
> system may already be doing other things during our probe.
> 
> There is a small possibility that some other driver that was a client
> of SPI_NOR didn't handle -EPROBE_DEFER and was relying on probe
> ordering and only worked when the SPI_NOR and the SPI bus were
> builtin.  In that case the other driver has a bug that's waiting to
> hit and the other driver should be fixed.

linux-next now triggers the following warning in kernel/kmod.c:136 on my
board. I've bisected this to this patch.

kmod.c:
        /*
         * We don't allow synchronous module loading from async.  Module
         * init may invoke async_synchronize_full() which will end up
         * waiting for this task which already is waiting for the module
         * loading to complete, leading to a deadlock.
         */
        WARN_ON_ONCE(wait && current_is_async());

[    1.849801] ------------[ cut here ]------------
[    1.854271] mscc_felix 0000:00:00.5: device is disabled, skipping
[    1.858753] WARNING: CPU: 1 PID: 7 at kernel/kmod.c:136 __request_module+0x3a4/0x568
[    1.858755] Modules linked in:
[    1.865028] fsl_enetc 0000:00:00.0: Adding to iommu group 1
[    1.872640] CPU: 1 PID: 7 Comm: kworker/u4:0 Not tainted 5.9.0-rc6-00001-g03edda0e1eda #113
[    1.872642] Hardware name: Kontron SMARC-sAL28 (Single PHY) on SMARC Eval 2.0 carrier (DT)
[    1.872647] Workqueue: events_unbound async_run_entry_fn
[    1.876013] spi-nor spi0.0: w25q32dw (4096 Kbytes)
[    1.881294] pstate: 00000005 (nzcv daif -PAN -UAO BTYPE=--)
[    1.881297] pc : __request_module+0x3a4/0x568
[    1.881299] lr : __request_module+0x39c/0x568
[    1.881302] sp : ffff8000113a3920
[    1.925739] x29: ffff8000113a3920 x28: ffff800010c7b000 
[    1.931068] x27: ffff00207ae05648 x26: ffff800010a41a88 
[    1.936397] x25: 0000000000000000 x24: 0000000000000000 
[    1.941727] x23: ffff800010c35140 x22: 0000000000000001 
[    1.947055] x21: ffff800011149948 x20: ffff800010615bdc 
[    1.952383] x19: 00000000ffffffff x18: 0000000000000000 
[    1.957447] fsl_enetc 0000:00:00.0: enabling device (0400 -> 0402)
[    1.957711] x17: ffff800010a3e618 x16: ffff800010a3e5f8 
[    1.964175] libphy: Freescale ENETC MDIO Bus: probed
[    1.969238] x15: ffffffffffffffff x14: ffff800011149948 
[    1.969241] x13: ffff8000113a3918 x12: 0000000000000018 
[    1.969245] x11: 0000000000000005 x10: 0101010101010101 
[    1.975241] 10 fixed-partitions partitions found on MTD device 20c0000.spi
[    1.979550] x9 : ffff80001005f6a4 x8 : 0000000000000000 
[    1.979553] x7 : 606f2c6364776865 x6 : 05041c090d431511 
[    1.979556] x5 : 1115430d091c0405 x4 : 0000000000000000 
[    1.979558] x3 : 6dac8d8d2dccae00 x2 : ffff800010c956e8 
[    1.979561] x1 : ffff80001005fa58 x0 : 0000000000000001 
[    1.979564] Call trace:
[    1.979571]  __request_module+0x3a4/0x568
[    1.984914] Creating 10 MTD partitions on "20c0000.spi":
[    1.990227]  parse_mtd_partitions+0x2ec/0x3c0
[    1.990232]  mtd_device_parse_register+0xdc/0x1c8
[    1.997133] 0x000000000000-0x000000010000 : "rcw"
[    2.002454]  spi_nor_probe+0x29c/0x2f0
[    2.002458]  spi_mem_probe+0x74/0xb0
[    2.017759] 0x000000010000-0x000000100000 : "failsafe bootloader"
[    2.018433]  spi_drv_probe+0x88/0xe8
[    2.018439]  really_probe+0xec/0x3c0
[    2.033744] 0x000000100000-0x000000140000 : "failsafe DP firmware"
[    2.035555]  driver_probe_device+0x60/0xc0
[    2.035559]  __device_attach_driver+0x8c/0xd0
[    2.040455] 0x000000140000-0x0000001e0000 : "failsafe trusted firmware"
[    2.044642]  bus_for_each_drv+0x84/0xd8
[    2.044645]  __device_attach_async_helper+0xc4/0xe8
[    2.044648]  async_run_entry_fn+0x4c/0x150
[    2.044653]  process_one_work+0x1f4/0x4b8
[    2.057751] 0x0000001e0000-0x000000200000 : "reserved"
[    2.062814]  worker_thread+0x50/0x480
[    2.062817]  kthread+0x160/0x168
[    2.062821]  ret_from_fork+0x10/0x34
[    2.073748] 0x000000200000-0x000000210000 : "configuration store"
[    2.076185] ---[ end trace 44224cc02e4e53d2 ]---

-michael