Re: [PATCH] usb: ehci: make HC see up-to-date qh/qtd descriptor ASAP

From: Marc Zyngier <marc.zyngier@arm.com>
To: Mark Salter <msalter@redhat.com>
Cc: Russell King <linux@arm.linux.org.uk>,
	Chen Peter-B29397 <B29397@freescale.com>,
	"ming.lei@canonical.com" <ming.lei@canonical.com>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
	Will Deacon <Will.Deacon@arm.com>,
	"stern@rowland.harvard.edu" <stern@rowland.harvard.edu>,
	Greg KH <greg@kroah.com>,
	"linux-omap@vger.kernel.org" <linux-omap@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] usb: ehci: make HC see up-to-date qh/qtd descriptor ASAP
Date: Wed, 31 Aug 2011 17:12:48 +0100	[thread overview]
Message-ID: <4E5E5D80.6080409@arm.com> (raw)
In-Reply-To: <1314804471.2344.79.camel@deneb.redhat.com>

On 31/08/11 16:27, Mark Salter wrote:
> On Wed, 2011-08-31 at 16:21 +0100, Will Deacon wrote:
>> On Wed, Aug 31, 2011 at 02:43:33PM +0100, Mark Salter wrote:
>>> On Wed, 2011-08-31 at 09:49 +0100, Will Deacon wrote:
>>>> On Wed, Aug 31, 2011 at 01:23:47AM +0100, Chen Peter-B29397 wrote:
>>>>> One question: why this write buffer issue did not happen at UP ARM V7 platform, whose dma buffer
>>>>> also uncache, but bufferable?
>>>>
>>>> Which CPU was on this platform?
>>>
>>> Using a 3.1.0-rc4+ kernel on a Pandaboard, and running 'hdparm -t' on a
>>> usb disk drive, I see ~5.8MB/s read speed. Same kernel, but passing
>>> nosmp on the commandline, I see 20.3MB/s.
>>>
>>> Can someone explain why nosmp would make such a difference?
>>
>> Oh gawd, that's horrible. I have a feeling it's probably a separate issue
>> though, caused by:
>>
>> omap_modify_auxcoreboot0(0x200, 0xfffffdff);
>>
>> in boot_secondary for OMAP. Unfortunately I have no idea what that line is
>> doing because it ends up talking to the secure monitor.
> 
> Okay, I may poke around a bit with that to see I can get a better
> understanding.
> 
> With the patched ehci-q.c, I see no noticeable difference between smp
> and nosmp. Both get me around 23.5MB/s with my setup.

Oddly enough, this patch doesn't do anything on my Tegra setup. In both
cases, I get around 17MB/s from a crap SD card plugged in a USB reader.

This leads me to suspect that this issue is very much OMAP4 specific.
Can anyone verify this theory on other some A9 platforms?

Cheers,

	M.
-- 
Jazz is not dead. It just smells funny...