The lengthy timeout previously used sometimes resulted in
scheduling problems, detailed below. Therefore reduce the timeout
to 500us. This timeout selection is supported by the benchmarks
collected below with various clock dividers. This is purely the time
spent polling (reported by ktime_get()).

div  1: max:150us avg:  2us
div  2: max:155us avg:  3us
div  4: max:149us avg:  7us
div  8: max:153us avg: 13us
div 16: max:197us avg: 21us
div 32: max:181us avg: 50us
div 64: max:262us avg:100us

Jan 22 01:27:21 rain27bmc kernel: rcu: INFO: rcu_sched self-detected stall on CPU
Jan 22 01:27:21 rain27bmc kernel: rcu:         0-....: (2099 ticks this GP) idle=0ca/1/0x40000002 softirq=349573/349573 fqs=1048
Jan 22 01:27:21 rain27bmc kernel:         (t=2100 jiffies g=841149 q=7163)
Jan 22 01:27:21 rain27bmc kernel: NMI backtrace for cpu 0
Jan 22 01:27:21 rain27bmc kernel: CPU: 0 PID: 5959 Comm: ibm-read-vpd Not tainted 5.8.17-a9b4ea8 #1
Jan 22 01:27:21 rain27bmc kernel: Hardware name: Generic DT based system
Jan 22 01:27:21 rain27bmc kernel: Backtrace:
Jan 22 01:27:25 rain27bmc kernel: [<8010d92c>] (dump_backtrace) from [<8010db80>] (show_stack+0x20/0x24)
...
Jan 22 01:27:25 rain27bmc kernel: [<8010130c>] (gic_handle_irq) from [<80100b0c>] (__irq_svc+0x6c/0x90)
Jan 22 01:27:25 rain27bmc kernel: Exception stack(0xb79159b0 to 0xb79159f8)
Jan 22 01:27:25 rain27bmc kernel: 59a0:                                     9e88e5d5 00000559 00000559 00000018
Jan 22 01:27:25 rain27bmc kernel: 59c0: 00000000 9f217c55 00000003 00000559 a0201c00 bfa4d048 bfa4d000 b7915a44
Jan 22 01:27:25 rain27bmc kernel: 59e0: 40e88f8a b7915a00 3254e553 80734924 80030113 ffffffff
Jan 22 01:27:25 rain27bmc kernel:  r9:b7914000 r8:a0201c00 r7:b79159e4 r6:ffffffff r5:80030113 r4:80734924
Jan 22 01:27:25 rain27bmc kernel: [<807348b4>] (__opb_read) from [<80734d98>] (aspeed_master_read+0xbc/0xcc)
Jan 22 01:27:25 rain27bmc kernel:  r10:00000004 r9:00000002 r8:80734cdc r7:bd33fa40 r6:00000004 r5:bd33f840
Jan 22 01:27:25 rain27bmc kernel:  r4:00201c00
Jan 22 01:27:25 rain27bmc kernel: [<80734cdc>] (aspeed_master_read) from [<807320f0>] (fsi_master_read+0x6c/0x1bc)
...
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20210211194846.35475-1-eajames@linux.ibm.comSigned-off-by: Joel Stanley <joel@jms.id.au>