linux-kernel - Re: [PATCH PoC 1/3] ACPI / PNP: Don't add enumeration_by

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Date:   Fri, 19 Aug 2022 13:41:56 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     John Garry <john.garry@...wei.com>
Cc:     Len Brown <lenb@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [PATCH PoC 1/3] ACPI / PNP: Don't add enumeration_by_parent devices

On Fri, Aug 19, 2022 at 1:20 PM John Garry <john.garry@...wei.com> wrote:
> On 19/08/2022 10:59, Andy Shevchenko wrote:
> >>>> The hisi_lpc driver also creates a platform device serial device for uart,
> >>>> which is the actual uart which we want to use - see
> >>>> hisi_lpc_acpi_add_child(). That function does not check
> >>>> physical_node_count value, but acpi_create_platform_device() does check it.
> >>>> So if we were to move hisi_lpc_acpi_add_child() across to use
> >>>> acpi_create_platform_device(), then the change in this patch is required to
> >>>> not create the PNP binding (so that physical_node_count is not set from
> >>>> PNP probe).
> >>> Hmm... The flag, as I interpret it, is equal to "the device in
> >>> question is a peripheral device to the non-discoverable bus, such as
> >>> SPI, I2C or UART". I.o.w. I do not see how PNP suits here. So, from my
> >>> point of view it seems like an abuse of the flag. Not sure the current
> >>> state of affairs in ACPI glue layer regarding this, though.
> >> Sorry, but I'm not following you here. Which flag are you talking about?
> > "enumerated by parent".
>
> ok, right. So I thought that PNP0501 was a standard cid to describe
> 16550-compat UART which may be on an LPC bus. And a LPC bus is
> non-discoverable, like SPI or I2C, which also use "enumerated by parent".

While in most cases the above-mentioned UART is sitting behind LPC and
what you described is all true, there is no requirement to have
PNP0501 to be tightened to LPC bus, so it can be any 8250-compatible
UART.

> Indeed, using PNP0501 for hisi lpc UART cid would be without problem,
> apart from the fact that we can't do all the PIO translation (so need
> "enumerated by parent").

Yeah, what can use this flag is the UART peer that connects to the
UART host, but it's a bit grey, because UART is p2p and not
controller-peripheral type of connections.

-- 
With Best Regards,
Andy Shevchenko