linux-kernel - RE: [PATCH] acpi: configfs: Unload SSDT on configfs entry removal

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Message-ID: <94F2FBAB4432B54E8AACC7DFDE6C92E37E5B8D42@ORSMSX110.amr.corp.intel.com>
Date:   Tue, 30 May 2017 21:16:48 +0000
From:   "Moore, Robert" <robert.moore@...el.com>
To:     Jan Kiszka <jan.kiszka@...mens.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
CC:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, "Zheng, Lv" <lv.zheng@...el.com>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "devel@...ica.org" <devel@...ica.org>
Subject: RE: [PATCH] acpi: configfs: Unload SSDT on configfs entry removal



> -----Original Message-----
> From: Jan Kiszka [mailto:jan.kiszka@...mens.com]
> Sent: Monday, May 29, 2017 5:53 AM
> To: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Cc: Rafael J. Wysocki <rjw@...ysocki.net>; Len Brown <lenb@...nel.org>;
> Zheng, Lv <lv.zheng@...el.com>; linux-acpi@...r.kernel.org; Linux Kernel
> Mailing List <linux-kernel@...r.kernel.org>; devel@...ica.org; Moore,
> Robert <robert.moore@...el.com>
> Subject: Re: [PATCH] acpi: configfs: Unload SSDT on configfs entry
> removal
> 
> On 2017-05-29 14:47, Mika Westerberg wrote:
> > On Mon, May 29, 2017 at 01:33:29PM +0200, Jan Kiszka wrote:
> >> Enhance acpi_load_table to also return the table index. Use that
> >> index to unload the table again when the corresponding directory in
> >> configfs gets removed. This allows to change SSDTs without rebooting
> the system.
> >> It also allows to destroy devices again that a dynamically loaded
> >> SSDT created.
> >>
> >> This is widely similar to the DT overlay behavior.
> >>
> >> Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
> >> ---
> >>
> >> Can someone explain to me why an unloaded table still sticks around
> >> in sysfs and why we cannot release its ID and rather have to use a
> >> new one when loading a modified version?
> >
> > IIRC ACPICA relies the fact that SSDTs are never unloaded. Bob (CC'd)
> > can correct me if I got it wrong.
> 


[Moore, Robert] 

I'm not entirely sure what the table manager code looks like at this time, but ACPICA does in fact support table unloading.

It is a rather dangerous thing to do, however -- unless you are careful about it. Basically, all handles that reference the table to be unloaded will go bad.

> OK... Is that standard-driven or just a limitation of this
> implementation?
> 
> Is there an upper limit of tables? I'm thinking of lengthy development
> sessions that play with tables, loading and unloading modified versions.
> 
 
[Moore, Robert] 

I think that the maximum number of loaded ACPI tables is 255 at any given time. However, things are cleaned up after an unload such that repeated load/unload cycles should not consume resources.


> Jan
> 
> --
> Siemens AG, Corporate Technology, CT RDA ITP SES-DE Corporate Competence
> Center Embedded Linux