Re: [PATCH v1 1/2] platform/x86: serial-multi-instantiate: Set fwnode for i2c

[Hans de Goede <hdegoede@redhat.com>]

Hi Stefan,

On 11/24/22 12:07, Stefan Binding wrote:
> This allows the i2c driver to obtain the ACPI_COMPANION.
> 
> Signed-off-by: Stefan Binding <sbinding@opensource.cirrus.com>
> ---
>  drivers/platform/x86/serial-multi-instantiate.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/platform/x86/serial-multi-instantiate.c b/drivers/platform/x86/serial-multi-instantiate.c
> index 5362f1a7b77c..15ef2f3c442e 100644
> --- a/drivers/platform/x86/serial-multi-instantiate.c
> +++ b/drivers/platform/x86/serial-multi-instantiate.c
> @@ -194,6 +194,7 @@ static int smi_i2c_probe(struct platform_device *pdev, struct smi *smi,
>  		strscpy(board_info.type, inst_array[i].type, I2C_NAME_SIZE);
>  		snprintf(name, sizeof(name), "%s-%s.%d", dev_name(dev), inst_array[i].type, i);
>  		board_info.dev_name = name;
> +		board_info.fwnode = acpi_fwnode_handle(adev);
>  
>  		ret = smi_get_irq(pdev, adev, &inst_array[i]);
>  		if (ret < 0)

I'm afraid that making this change is not as straight forward as it looks.

I know that I have tried to do this in the past and it failed.

IIRC there were 3 problems:

1. I was expecting this to also allow the driver for the instantiated
i2c-client to be able to bind using an acpi_match_table but that
unfortunately does not work. acpi_match_table matches only work for
the first physical_node linked under
/sys/bus/acpi/devices/xxxx:xx/physical_node and that is the platform
device to which serial-multi-instantiate.c binds. The i2c_client becomes
the second physical node.  Note this is not really an issue,
just something to be aware of.


2. This causes the i2c-core to use the first IRQ resource in the ACPI
fwnode as client->irq for any clients for which we do not set an
IRQ when instantiating. Which may very well be wrong. Sometimes that
IRQ is only valid for the first i2c-client which we instantiate; and
not for the others! And sometimes it is a problem because it may
point to an irqchip for which we never wrote a driver leading to
all probes of the i2c-client failing with -EPROBE_DEFER, see:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d1d84bb95364ed604015c2b788caaf3dbca0262f

Note that patch has been reverted since that specific -EPROBE_DEFER
issue has been solved by making the ACPI core instantiate a
platform_device instead of an i2c_client (in this case we
did not need the actual i2c_client at all).

The current i2c-core code has a (!client-irq) test guarding its
code of trying to use the first ACPI fwnode IRQ resource.

So we could disable this by setting client->irq = -ENOENT in
serial-multi-instantiate.c when (inst->flags & IRQ_RESOURCE_TYPE) ==
IRQ_RESOURCE_NONE). But that will introduce a new problem. Many
i2c-drivers check if there is an IRQ for them to use by doing:
"if (client->irq) request_irq(client->irq, ...)" but then with
error checking/so setting client->irq to -ENOENT will cause
the request_irq to fail, leading the probe to fail.

So before you can write a patch setting client->irq = -ENOENT
when (inst->flags & IRQ_RESOURCE_TYPE) == IRQ_RESOURCE_NONE),
you would first need to patch all i2c-drivers for clients
instantiated through serial-multi-instantiate.c changing:

	if (client->irq) {
		...
	}

to:

	if (client->irq > 0) {
		...
	}

Note this is not as bad as it sounds, since there are only
a few drivers for clients instantiated by serial-multi-instantiate.c .


3. Some drivers may check for an ACPI companion device and then
change their behavior. So all drivers for clients instantiated
through serial-multi-instantiate.c will need to be audited for
this (and a summary of this audit needs to be added to the commit
msg).

Regards,

Hans