Re: [PATCH v2 00/13] OMAP mailbox FIFO removal

[Dominic Rath]

On 13.06.2024 14:22, Andrew Davis wrote:
> > We looked into this some time ago, and noticed that the IRQ approach 
> > caused problems in the virtio/rpmsg code. I'd like to understand if 
> > your change was for the same reason, or something else we missed before.
>
> It is most likely the same reason. Seems despite its name, 
> rproc_vq_interrupt() cannot
> be called from an IRQ/atomic context. As the following backtrace shows, 
> that function
> calls down into functions which are not IRQ safe. So we needed to keep 
> it threaded:
>
> [    5.389374] BUG: scheduling while atomic: (udev-worker)/232/0x00010002
> [    5.395917] Modules linked in: videobuf2_dma_contig videobuf2_memops 
> videobuf2_v4l2 phy_j721e_wiz display_connector omap_mailbox(+) videodev 
> tps6594_i2c(+) videobuf2_common phy_can_transceiver at24 cd6
> [    5.433562] CPU: 0 PID: 232 Comm: (udev-worker) Not tainted 
> 6.10.0-rc1-next-20240528-dirty #10
> [    5.442158] Hardware name: Texas Instruments AM69 SK (DT)
> [    5.447540] Call trace:
> [    5.449976]  dump_backtrace+0x94/0xec
> [    5.453640]  show_stack+0x18/0x24
> [    5.456944]  dump_stack_lvl+0x78/0x90
> [    5.460598]  dump_stack+0x18/0x24
> [    5.463900]  __schedule_bug+0x50/0x68
> [    5.467552]  __schedule+0x80c/0xb0c
> [    5.471029]  schedule+0x34/0x104
> [    5.474243]  schedule_preempt_disabled+0x24/0x40
> [    5.478845]  rwsem_down_write_slowpath+0x31c/0x56c
> [    5.483622]  down_write+0x90/0x94
> [    5.486924]  kernfs_add_one+0x3c/0x148
> [    5.490661]  kernfs_create_dir_ns+0x50/0x94
> [    5.494830]  sysfs_create_dir_ns+0x70/0x10c
> [    5.498999]  kobject_add_internal+0x98/0x26c
> [    5.503254]  kobject_add+0x9c/0x10c
> [    5.506729]  device_add+0xc0/0x790
> [    5.510120]  rpmsg_register_device_override+0x10c/0x1c0
> [    5.515333]  rpmsg_register_device+0x14/0x20
> [    5.519590]  virtio_rpmsg_create_channel+0xb0/0x104
> [    5.524452]  rpmsg_create_channel+0x28/0x60
> [    5.528622]  rpmsg_ns_cb+0x100/0x1dc
> [    5.532185]  rpmsg_recv_done+0x114/0x2e4
> [    5.536094]  vring_interrupt+0x68/0xa4
> [    5.539833]  rproc_vq_interrupt+0x2c/0x48
> [    5.543830]  k3_r5_rproc_mbox_callback+0x84/0x90 [ti_k3_r5_remoteproc]
> [    5.550348]  mbox_chan_received_data+0x1c/0x2c
> [    5.554779]  mbox_interrupt+0xa0/0x17c [omap_mailbox]
> [    5.559820]  __handle_irq_event_percpu+0x48/0x13c
> [    5.564511]  handle_irq_event+0x4c/0xac

I looked into this a bit more closely, together with the colleague who 
implemented our internal workaround, and we came up with one more concern:

Have you considered that this synchronous path from the (threaded) IRQ 
draining the mailbox down to the (potentially blocking) rpmsg_* calls 
might let the hardware mailbox grow full?

From what I remember the vring (?) has room for 512 messages, but the 
hardware mailbox on e.g. the AM64x can only handle four messages. At 
least with the current implementation on TI's MCU+ SDK running on the 
R5f that could cause the R5f to block, waiting for room in the hardware 
mailbox, while there are plenty of vring buffers available.

Best Regards,

Dominic