RE: [PATCH] dmaengine: pl330: issue_pending waits until WFP state

["bumyong.lee" <bumyong.lee@samsung.com>]

Hello.

> On Thu, Dec 21, 2023 at 10:00:26PM +0530, Vinod Koul wrote:
> 
> This seems to cause a stall on my Quartz 64 model B (RK3566) once
> Bluetooth over UART is initialized, when combined with a patch of mine
> that enables DMA on UARTs [1]. Reverting this patch gets everything
> running again.
> 
> The following are RCU stalls detected, followed by stack traces produced
> with pseudo-NMI. Without pseudo-NMIs no stack traces are produced.
> 
>     rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
>     rcu:     0-...0: (0 ticks this GP) idle=80fc/1/0x4000000000000000
> softirq=693/693 fqs=31498
>     rcu:     3-...0: (3 ticks this GP) idle=2b44/1/0x4000000000000000
> softirq=553/556 fqs=31498
>     rcu:     (detected by 1, t=162830 jiffies, g=-307, q=32 ncpus=4)
>     Sending NMI from CPU 1 to CPUs 0:
>     NMI backtrace for cpu 0
>     CPU: 0 PID: 1200 Comm: (udev-worker) Not tainted 6.7.0-rc6-next-
> 20231222-10300-g8b07e3811bc7 #17
>     Hardware name: Pine64 RK3566 Quartz64-B Board (DT)
>     pstate: 00400009 (nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>     pc : queued_spin_lock_slowpath+0x50/0x330
>     lr : pl330_irq_handler+0x2f8/0x5a0
>     sp : ffffffc080003ec0
>     pmr_save: 00000060
>     x29: ffffffc080003ec0 x28: ffffff80017c7000 x27: ffffff8001a58d80
>     x26: 0000000000000060 x25: ffffff80017d0338 x24: ffffff800161ae38
>     x23: ffffff8001597c00 x22: ffffffc081960000 x21: 0000000000000000
>     x20: ffffff800161ac80 x19: ffffff80010c5180 x18: 0000000000000000
>     x17: ffffffc06e724000 x16: ffffffc080000000 x15: 0000000000000000
>     x14: 0000000000000000 x13: ffffff80042f102f x12: ffffffc083ad3cc4
>     x11: 0000000000000040 x10: ffffff800022a0a8 x9 : ffffff800022a0a0
>     x8 : ffffff8000400270 x7 : 0000000000000000 x6 : 0000000000000000
>     x5 : ffffff8000400248 x4 : ffffffc06e724000 x3 : ffffffc080003fa0
>     x2 : 0000000000000000 x1 : 0000000000000001 x0 : ffffff800161ae38
>     Call trace:
>      queued_spin_lock_slowpath+0x50/0x330
>      __handle_irq_event_percpu+0x38/0x16c
>      handle_irq_event+0x44/0xf8
>      handle_fasteoi_irq+0xb0/0x28c
>      generic_handle_domain_irq+0x2c/0x44
>      gic_handle_irq+0x10c/0x240
>      call_on_irq_stack+0x24/0x4c
>      do_interrupt_handler+0x80/0x8c
>      el1_interrupt+0x44/0x98
>      el1h_64_irq_handler+0x18/0x24
>      el1h_64_irq+0x78/0x7c
>      __d_rehash+0x0/0x94
>      d_add+0x40/0x80
>      simple_lookup+0x4c/0x78
>      path_openat+0x5ec/0xed0
>      do_filp_open+0x80/0x12c
>      do_sys_openat2+0xb4/0xe8
>      __arm64_sys_openat+0x64/0xa4
>      invoke_syscall+0x48/0x114
>      el0_svc_common.constprop.0+0x40/0xe0
>      do_el0_svc+0x1c/0x28
>      el0_svc+0x34/0xd4
>      el0t_64_sync_handler+0x100/0x12c
>      el0t_64_sync+0x1a4/0x1a8
>     Sending NMI from CPU 1 to CPUs 3:
>     NMI backtrace for cpu 3
>     CPU: 3 PID: 31 Comm: kworker/3:0 Not tainted 6.7.0-rc6-next-20231222-
> 10300-g8b07e3811bc7 #17
>     Hardware name: Pine64 RK3566 Quartz64-B Board (DT)
>     Workqueue: events hci_uart_write_work [hci_uart]
>     pstate: 80400009 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>     pc : _state+0x2c/0x138
>     lr : pl330_start_thread.isra.0+0x2e0/0x32c
>     sp : ffffffc08157bb20
>     pmr_save: 00000060
>     x29: ffffffc08157bb20 x28: 0000000000000000 x27: 0000000000000060
>     x26: ffffffc080c4e658 x25: 0000000000000060 x24: 0000000001a20000
>     x23: ffffff8001555000 x22: ffffffc081960020 x21: ffffff800161b068
>     x20: 0000000000000000 x19: ffffff800161b050 x18: ffffffffffffffff
>     x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000001
>     x14: 0000000000000004 x13: 0000000000000009 x12: 0000000000000005
>     x11: 0000000000000027 x10: 000000000000002b x9 : 0000000000000032
>     x8 : ffffffc08154521d x7 : 0000000000000005 x6 : 0000000000000010
>     x5 : 0000000000000001 x4 : ffffffc081960d04 x3 : ffffff800161b280
>     x2 : ffffff800161b050 x1 : 0000000000204000 x0 : 0000000000000108
>     Call trace:
>      _state+0x2c/0x138
>      pl330_tasklet+0x1f8/0x818
>      pl330_issue_pending+0x150/0x178
>      serial8250_tx_dma+0x150/0x21c
>      serial8250_start_tx+0x9c/0x1c0
>      __uart_start+0x74/0xfc
>      uart_write+0xfc/0x2f0
>      ttyport_write_buf+0x4c/0x90
>      serdev_device_write_buf+0x24/0x38
>      hci_uart_write_work+0x54/0x164 [hci_uart]
>      process_one_work+0x13c/0x2bc
>      worker_thread+0x2a0/0x52c
>      kthread+0xe0/0xe4
>      ret_from_fork+0x10/0x20
I think the dma channel already passed WFP state.

The errata[1] says that
4.  The driver polls the status of channel 0 until it observes that
     it has entered the "Waiting for peripheral" state 
5.  The driver enables the peripheral to allow it to issue DMA requests

When I review 8250_dma.c, I think serial8250_do_prepare_tx_dma() of 
serial8250_tx_dma() would enable peripheral to allow issue DMA requests.
serial8250_do_prepare_tx_dma(p) performs before dma_async_issue_pending()

It means that serial8250_tx_dma() has reversed sequence step 4 and 5 for
pl330.

But I'm not sure if the errata suggests normal slave driver dma usage
sequence or not.

If it is abnormal sequence, I think it should be handled by quirk
[1]: https://developer.arm.com/documentation/genc008428/latest