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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