Conversation

Takashi Sakamoto (坂本 貴史)

I sent:

[GIT PULL] firewire updates for v6.12
https://lore.kernel.org/lkml/20240921031421.GA227826@workstation.local/

The batch of changes includes the followwing:

- Replacing tasklet with usual workqueue for isochronous context
- Replacing IDR with XArray
- Utilizing guard macro where possible
- Printing deprecation warning when enabling debug parameter of
firewire-ohci module

Additionally, it includes a single patch for sound subsystem which the
subsystem maintainer acked:

- Switching to nonatomic PCM operation

In FireWire subsystem, tasklet has been used as the bottom half of 1394
OHCi hardIRQ so long. In the recent kernel updates, BH workqueue has
been available, and some developers have proposed replacing tasklet with
BH workqueue. While it is fortunate that developers are still considering
the legacy subsystem, a simple replacement is not necessarily suitable.

As a first step towards dropping tasklet, I've investigated the
feasibility for 1394 OHCI isochronous context, and concluded that usual
workqueue is available. In the context, the batch of packets is processed
in the specific queue, thus the timing jitter caused by task scheduling is
not so critical. Additionally, DMA transmission can be scheduled
per-packet basis, therefore the context can be sleep between the operation
of transmissions. Furthermore, in-kernel protocol implementation involves
some CPU-bound tasks, which can sometimes consumes CPU time so long. These
characteristics suggest that usual workqueue is suitable, through BH
workqueues are not.

The replacement with usual workqueue allows unit drivers to process the
content of packets in non-atomic context. It brings some reliefs to some
drivers in sound subsystem that spin-lock is not mandatory anymore during
isochronous packet processing.
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