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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