linux-kernel - Re: Q: volatile vs barriers to access memory data changed by device DMA

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Message-ID: <20080225083201.51067611@core>
Date:	Mon, 25 Feb 2008 08:32:01 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Eliot Blennerhassett <linux@...ioscience.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Q: volatile vs barriers to access memory data changed by device
 DMA

> === after conversion
> struct bus_master_interface *interface;
> while (interface->ack != OK) { 
>         delay(a short while);
>         rmb(); 
>         [ after X loops device changes interface->ack by dma ]
> };
> 
> All I need is for the read of interface->ack in the loop not to be optimised 
> away - is rmb() the appropriate incantation to achieve this?

Yes - ish.  You want the equivalent of 

	do {
		rmb();
		if (interface->ack == OK)
			break;
	} while(1);

(eg putting another rmb before the while in your case)

You want a barrier before the *first* read in case the
compiler has managed to cache the value before you enter the loop.


> struct bus_master_interface *interface;
> interface->cmd = command;
> wmb();
> iowrite(device_interrupt, 1);

Yes.

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