linux-kernel - Re: [PATCH] USB: host: ehci: allow tine of highspeed nak-count

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Date:   Fri, 16 Nov 2018 12:53:57 +0000
From:   Ben Dooks <ben.dooks@...ethink.co.uk>
To:     Alan Stern <stern@...land.harvard.edu>,
        Ben Dooks <ben-linux@...ff.org>
Cc:     gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
        linux-kernel@...ts.codethink.co.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: host: ehci: allow tine of highspeed nak-count

On 14/11/18 18:47, Alan Stern wrote:
> On Wed, 14 Nov 2018, Ben Dooks wrote:
> 
>> From: Ben Dooks <ben.dooks@...ethink.co.uk>
>>
>> At least some systems benefit with less scheduling if the NAK count
>> value is set higher than the default 4. For instance a Tegra3 with
>> an SMSC9512 showed less interrupt load when this was changed to 14.
> 
> Interesting.  Why do you think this is?  In theory, increasing the NAK
> count RL value should cause a higher memory bus load and perhaps a
> slight rise in the interrupt load (transfers will complete a little
> more quickly because the controller tries harder to poll the endpoints
> and see if they are ready).

I thought the NAK counter was decremented until the transfer is given
up on. I'm going to have to go back and get some actual figures from
a running system as this was originally done over a year ago with the
SMSC9512 (IIRC) network driver.

>> To allow the changing of this value, add a sysfs node to each of
>> the controllers to allow run-time changing.
>>
>> Signed-off-by: Ben Dooks <ben.dooks@...ethink.co.uk>
> 
> The patch looks okay to me.

I'll look at getting some tracing from the SMSC driver to see what
is going on.


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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