Re: [PATCH 4/8] i386: bitops: Kill volatile-casting of memory addresses

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Linus Torvalds wrote:
And before we remove that "volatile", we'd better make damn sure that there isn't any driver that does
	/* Wait for the command queue to be cleared by DMA */
	while (test_bit(...))
		;

or similar.

Yes, it's annoying, but this is a scary and subtle area. And we sadly _have_ had code that does things like that.

I certainly cannot speak for all the grotty drivers out there, but I've never ever seen anything like the above. I would consider anyone using kernel bit operations on DMA memory to be more than a little crazy. But that's just me :)
Usually you will see

	while (1) {
		rmb();
		if (software_index == hardware_index_in_DMA_memory)
			break;
		
		... handle a unit of work ...
	}

Though ISTR being told that even rmb() was not sufficient in all cases [nonetheless, that's what I use in net and SATA drivers and email recommendations, and people seem happy]
	Jeff


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