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

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Date:	Thu, 23 May 2013 21:04:53 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Takashi Iwai <tiwai@...e.de>
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.

On Thu, May 23, 2013 at 8:05 PM, Takashi Iwai <tiwai@...e.de> wrote:
> 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.

Suppose it is, it is the fact, and it isn't OK to break this distribution.

Also udev supports user-defined rules to load firmware, which
means some drivers may not put their firmware in the default
path of distribution's firmware.

>
>> 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

Simply dropping the old way may cause user space regression.

> firmware file is loaded via user mode helper (except for explicit
> cases like FW_ACTION_NOHOTPLUG), for example.

As it is a very driver specific problem, it is better to solve it inside driver.

>
>
>> >> 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.

I don't think the way is too difficult to implement. In the path which
requires synchronization, it can be waited on one completion after
calling request_firmware_nowait().


Thanks,
-- 
Ming Lei
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