linux-kernel - Re: microcode loading got really slow.

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Message-ID: <s5hmwrlx5np.wl%tiwai@suse.de>
Date:	Thu, 23 May 2013 14:05:46 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Dave Jones <davej@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, x86@...nel.org,
	fenghua.yu@...el.com
Subject: Re: microcode loading got really slow.

At Thu, 23 May 2013 18:45:29 +0800,
Ming Lei wrote:
> 
> On Thu, May 23, 2013 at 6:36 PM, Takashi Iwai <tiwai@...e.de> wrote:
> >
> > No, f/w loader always fall back to user mode helper, as long as its
> > support is built in.  And doing that for microcode driver in that code
> > path isn't only superfluous but also broken due to request_firmware
> > call in module init.
> 
> Firstly, it is not good to do this since some distributions doesn't support
> direct loading and doesn't have udevd(such as, android).
> 
> Secondly, returning failure from request_firmware_direct() doesn't mean
> the firmware doesn't exist since distribution may put the firmware other where.

Right, the non-standard path is the problem, and basically the only
problem.  The distribution that doesn't support the direct loading
means nothing but that.

> Anyway, this example is very specific(no firmware can be accepted), and
> request_firmware_nowait() should be OK for the situation.

Oh no, rewriting with request_firmware_nowait() should be really the
last choice.  It would change the code flow awfully bad in most
cases.

The new kernel driver has a better firmware mechanism.  If it's only
the question of paths, we should move on toward that direction and
drop the too complex old way.  I'd vote for a warning shown when a
firmware file is loaded via user mode helper (except for explicit
cases like FW_ACTION_NOHOTPLUG), for example.

> >> wrt. this problem, I think we
> >> need to know why the direct loading is failed.
> >
> > The reason is obvious: the requested f/w file doesn't exist.
> > And it's fine, because the microcode update is an optional operation.
> > If no f/w file is found, it's not handled as an error.  It just means
> > that no need to update, continuing to work.
> 
> OK, as said above, the example is very specific, and might be
> workarounded by request_firmware_nowait().

It's not that easy in this case.  The microcode loader driver core
module doesn't invoke request_firmware() directly but it's via cpu
driver.  And the same callback is called in different code paths, not
only at init but also via sysfs write.  Thus the request_firmware()
call must be synchronous there.

Takashi
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