Messages in this thread | | | Date | Mon, 25 Feb 2008 14:04:01 +1300 | From | Eliot Blennerhassett <> | Subject | Q: volatile vs barriers to access memory data changed by device DMA |
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Greetings,
Currently I have a driver that uses "volatile", which I want to remove. As others have said "volatile is useless" Heres the relevant source. http://hg.alsa-project.org/alsa-driver/file/89222d702376/pci/asihpi/hpi6205.c
There's quite a bit written about barriers, but most seems to be assuming SMP situation or memory mapped devices. Not much about devices doing DMA. I.e I have read Documentation/memory-barriers.txt, and some of the threads in lkml, but still am unsure.
The "volatile" is applied to structures that are either read or written by device DMA. Certainly the driver in its current state doesn't work without volatile qualifier. (BTW the device doesn't use host interrupts)
Now, I want to get rid of the volatile, and replace it with ?some kind of barrier?
In the following, am I using the barriers correctly?
Note that structures ("interface") used for dma are allocated with dma_alloc_coherent()
1) Reading something updated by DMA Here the volatile or barrier is needed or the loop gets optimised away.
=== current code volatile struct bus_master_interface *interface; while (interface->ack != OK) { delay(a short while) [ after X loops device changes interface->ack by 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?
2) Writing to memory, interrupt device Need command to be in memory for device to read by DMA before device gets interrupted.
=== current code === volatile struct bus_master_interface *interface; interface->cmd = command; iowrite(device_interrupt, 1); [device reads interface->cmd by dma]
=== after conversion === struct bus_master_interface *interface; interface->cmd = command; wmb(); iowrite(device_interrupt, 1); [device reads interface->cmd by dma]
Is the wmb() a guarantee that the command will be in memory visible to the device when the driver informs it of a new command? Is it even needed? I.e. does iowrite() effectively form a barrier?
regards
-- Eliot Blennerhassett www.audioscience.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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