linux-kernel - Re: USB mass storage and ARM cache coherency

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Message-Id: <201002171010.57464.oliver@neukum.org>
Date:	Wed, 17 Feb 2010 10:10:57 +0100
From:	Oliver Neukum <oliver@...kum.org>
To:	"Shilimkar, Santosh" <santosh.shilimkar@...com>
Cc:	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Pavel Machek <pavel@....cz>, Greg KH <greg@...ah.com>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Ming Lei <tom.leiming@...il.com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel" <linux-arm-kernel@...ts.infradead.org>,
	"Mankad, Maulik Ojas" <x0082077@...com>,
	"Gadiyar, Anand" <gadiyar@...com>
Subject: Re: USB mass storage and ARM cache coherency

Am Mittwoch, 17. Februar 2010 09:55:08 schrieb Shilimkar, Santosh:
> > Your original patch however kills ehci, ohci and uhci on some architectures.
> 
> How about below approach? Controller driver can set 
> "uses_pio_for_control" if it can't do dma for control transfer.
> 
> diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
> index 80995ef..e3eae02 100644
> --- a/drivers/usb/core/hcd.c
> +++ b/drivers/usb/core/hcd.c
> @@ -1276,7 +1276,7 @@ static int map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
>  
>         if (usb_endpoint_xfer_control(&urb->ep->desc)
>             && !(urb->transfer_flags & URB_NO_SETUP_DMA_MAP)) {
> -               if (hcd->self.uses_dma) {
> +               if (hcd->self.uses_dma && !hcd->self.uses_pio_for_control) {

It is not elegant to describe exceptions. It would be better, if you split up
the flag into two flags, called uses_dma_for_ordinary_transfers and
uses_dma_for control_transfers. Doing so also makes sure you look at
all hcd drivers ;-)

And the tests become straightforward. And please add a detailed comment
to explain why this differentiation is needed on ARM.

	Regards
		Oliver
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