linux-kernel - Re: [PATCH 0/3] firewire: assist unit driver to compute packet timestamp

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Date:   Fri, 7 Jan 2022 20:01:18 +0900
From:   Takashi Sakamoto <o-takashi@...amocchi.jp>
To:     stefanr@...6.in-berlin.de, alsa-devel@...a-project.org,
        linux1394-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, marcan@...can.st
Subject: Re: [PATCH 0/3] firewire: assist unit driver to compute packet
 timestamp

Hi Stefan,

Wishing you a happy new year.

We are in the last week for release of v5.16 kernel, and soon merge
window for v5.17 kernel will be opened if thing goes well. I wish any
action for the review process to merge these patches into upstream.


Thanks

Takashi Sakamoto

On Tue, Dec 21, 2021 at 07:54:42PM +0900, Takashi Sakamoto wrote:
> Hi Stefan,
> 
> Thank you for your long effort to maintain Linux FireWire subsystem. I'd
> like to use the timestamp function for my integration in ALSA firewire
> stack planned at next version of Linux kernel. I'm glad if getting to
> your help for upstreaming.
> 
> On Thu, Dec 02, 2021 at 08:34:54PM +0900, Takashi Sakamoto wrote:
> > Hi,
> > 
> > In 1394 OHCI specification, each descriptor of IR/IT/AR/AT DMA context
> > has timeStamp field. The value of timeStamp field express the time in
> > which the controller accept packet. The resolution of value is isochronous
> > cycle count (8,000 Hz) with second up to 7.
> > 
> > I have a plan to use the value of timeStamp field for ALSA firewire stack
> > so that userspace ALSA PCM/Rawmidi applications can get converted timestamp
> > (ktime) for PCM frame/MIDI message. The timestamp can ideally express
> > finer granularity than the time to invoke IRQ handler (and co).
> > 
> > Current implementation of Linux FireWire subsystem delivers the value of
> > timeStamp field to unit driver for IR/IT/AT DMA context, but not for AR
> > DMA context. Additionally, the way to refer to Isochronous Cycle Timer
> > Register in MMIO region of 1394 OHCI controller is transaction to local
> > node. It includes overhead of transaction and it's preferable to add
> > less-overhead way available in any type of IRQ context.
> > 
> > This patchset adds two functions exposed in kernel space:
> > 
> >  * fw_card_read_cycle_time()
> >     * allow unit driver to access to CYCLE_TIME register in MMIO region
> >       without initiate transaction
> >  * fw_request_get_timestamp()
> >     * allow unit driver to get timestamp of request packet inner request
> >       handler
> > 
> > I note that Hector Martin found kernel null pointer dereference during
> > process to remove PCI card and has posted a patch:
> > 
> >  * https://lore.kernel.org/lkml/20211027113130.8802-1-marcan@marcan.st/
> > 
> > His patch is included in the series with my comment for relevant commit
> > 20802224298c ("firewire: core: add forgotten dummy driver methods, remove
> > unused ones"). The patch is required since unit driver can refer to dummy
> > driver between removal callback of PCI subsystem and removal callback of
> > FireWire subsystem.
> > 
> > Hector Martin (1):
> >   firewire: Add dummy read_csr/write_csr functions
> > 
> > Takashi Sakamoto (2):
> >   firewire: add kernel API to access CYCLE_TIME register
> >   firewire: add kernel API to access packet structure in request
> >     structure for AR context
> > 
> >  drivers/firewire/core-card.c        | 39 +++++++++++++++++++++++++++++
> >  drivers/firewire/core-cdev.c        |  6 +++--
> >  drivers/firewire/core-transaction.c | 18 +++++++++++++
> >  include/linux/firewire.h            |  3 +++
> >  4 files changed, 64 insertions(+), 2 deletions(-)
> > 
> > -- 
> > 2.32.0
> 
> 
> Sincerely yours
> 
> Takashi Sakamoto