linux-kernel - Re: [PATCH v2] ata: add AMD Seattle platform driver

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Date:	Mon, 01 Feb 2016 21:14:17 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Brijesh Singh <brijesh.singh@....com>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	hdegoede@...hat.com, linux-ide@...r.kernel.org,
	Graeme Gregory <graeme@...a.org.uk>
Subject: Re: [PATCH v2] ata: add AMD Seattle platform driver

On Monday 01 February 2016 12:56:06 Brijesh Singh wrote:
> On 01/29/2016 03:22 PM, Arnd Bergmann wrote:
> > 
> > For the ACPI case, I still think that an AML call from the AHCI driver
> > is the most logical solution. You mentioned that you believe that calling
> > into the AML interpreter up to 100 times per second is a noticeable
> > overhead, but I doubt that and would like to see actual number backing
> > that up. Note that most of the time, the status of the LEDs won't even
> > change, so the driver does not have to call into the AML while I/O
> > is in progress, or while it is stopped, only for the transition or in
> > case of locate and fault events that should be extremely rare.
> >
> During disk activity ahci_sw_activity_blink() is called based on timer expiration (~100ms).
> I just enabled the function profiler for 'dd if=/dev/zero of=/root/tmp bs=1M count=4096' and see the output below. The function was called 37 times in 2.5s
>  
> #echo 1 > function_profile_enabled
> 
> #dd if=/dev/zero of=/root/tempfile bs=1M count=4096
> 4096+0 records in
> 4096+0 records out
> 4294967296 bytes (4.3 GB) copied, 2.57334 s, 1.7 GB/s
> 
> #echo 0 > function_profile_enabled
> 
> #cat trace_stat/*
> Function                               Hit    Time            Avg             s^2
> --------                               ---    ----            ---             ---
> seattle_transmit_led_message            37    25.088 us       0.678 us        0.050 us    
> 
> I am not debating on your AML call recommendation, it sounds like a good idea however BIOS is already released hence its bit late to add AML methods for this. I am seeking guidance on what can be done in the given situation. I thought platform driver is one option to get this feature enabled in kernel.

This is where we really need the ACPI maintainers to explain the
general policy for dealing with firmware updates.

I would assume that adding the feature in a later firmware version
is a compatible change, and the feature is non-essential (the
device will work fine with the generic SATA driver, except
the LEDs don't blink), so it's not a big deal, it's just what
you get for having the firmware shipped before the driver is
reviewed (don't do that).

	Arnd