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

[Ahmad Fatoum]

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?

Cheers,
Ahmad

> 
> -Saravana
> 
> 


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