Re: [PATCH v2 2/2] of: base: Avoid console probe delay when fw_devlink.strict=1

[Saravana Kannan]

On Thu, Jun 23, 2022 at 10:36 AM Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx> wrote:
>
> Hello Saravana,
>
> On 23.06.22 19:26, Saravana Kannan wrote:
> > On Thu, Jun 23, 2022 at 3:05 AM sascha hauer <sha@xxxxxxxxxxxxxx> wrote:
> >>
> >> On Thu, Jun 23, 2022 at 01:03:43AM -0700, Saravana Kannan wrote:
> >>> Commit 71066545b48e ("driver core: Set fw_devlink.strict=1 by default")
> >>> enabled iommus and dmas dependency enforcement by default. On some
> >>> systems, this caused the console device's probe to get delayed until the
> >>> deferred_probe_timeout expires.
> >>>
> >>> We need consoles to work as soon as possible, so mark the console device
> >>> node with FWNODE_FLAG_BEST_EFFORT so that fw_delink knows not to delay
> >>> the probe of the console device for suppliers without drivers. The
> >>> driver can then make the decision on where it can probe without those
> >>> suppliers or defer its probe.
> >>>
> >>> Fixes: 71066545b48e ("driver core: Set fw_devlink.strict=1 by default")
> >>> Reported-by: Sascha Hauer <sha@xxxxxxxxxxxxxx>
> >>> Reported-by: Peng Fan <peng.fan@xxxxxxx>
> >>> Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>
> >>> Tested-by: Peng Fan <peng.fan@xxxxxxx>
> >>> ---
> >>>  drivers/of/base.c | 2 ++
> >>>  1 file changed, 2 insertions(+)
> >>>
> >>> diff --git a/drivers/of/base.c b/drivers/of/base.c
> >>> index d4f98c8469ed..a19cd0c73644 100644
> >>> --- a/drivers/of/base.c
> >>> +++ b/drivers/of/base.c
> >>> @@ -1919,6 +1919,8 @@ void of_alias_scan(void * (*dt_alloc)(u64 size, u64 align))
> >>>                       of_property_read_string(of_aliases, "stdout", &name);
> >>>               if (name)
> >>>                       of_stdout = of_find_node_opts_by_path(name, &of_stdout_options);
> >>> +             if (of_stdout)
> >>> +                     of_stdout->fwnode.flags |= FWNODE_FLAG_BEST_EFFORT;
> >>
> >> The device given in the stdout-path property doesn't necessarily have to
> >> be consistent with the console= parameter. The former is usually
> >> statically set in the device trees contained in the kernel while the
> >> latter is dynamically set by the bootloader. So if you change the
> >> console uart in the bootloader then you'll still run into this trap.
> >>
> >> It's problematic to consult only the device tree for dependencies. I
> >> found several examples of drivers in the tree for which dma support
> >> is optional. They use it if they can, but continue without it when
> >> not available. "hwlock" is another property which consider several
> >> drivers as optional. Also consider SoCs in early upstreaming phases
> >> when the device tree is merged with "dmas" or "hwlock" properties,
> >> but the corresponding drivers are not yet upstreamed. It's not nice
> >> to defer probing of all these devices for a long time.
> >>
> >> I wonder if it wouldn't be a better approach to just probe all devices
> >> and record the device(node) they are waiting on. Then you know that you
> >> don't need to probe them again until the device they are waiting for
> >> is available.
> >
> > That actually breaks things in a worse sense. There are cases where
> > the consumer driver is built in and the optional supplier driver is
> > loaded at boot. Without fw_devlink and the deferred probe timeout, we
> > end up probing the consumer with limited functionality. With the
> > current setup, sure we delay some probes a bit but at least everything
> > works with the right functionality. And you can reduce or remove the
> > delay if you want to optimize it.
>
> I have a system that doesn't use stdout-path and has the bootloader
> set console= either to ttynull when secure booting or to an UART
> when booting normally. How would I optimize the kernel to avoid
> my UART being loaded after DMA controller probe without touching
> the bootloader?
>

Thanks for the report Ahmad. I think someone else reported a similar
thing in another thread. I plan to take a look at it. It should be
possible to find the device and set the flag for those devices too.

-Saravana