Generic ATA driver DMA coherency issues

[Piotr Zięcik]

While working on bring-up SATA on ARM SoC I had issues with DMA memory 
coherency. After tracing, I have found problem in generic ATA code behaviour.

The ATA generic driver allocates 64k DMA workspace memory which can be 
optionally used by ATA chipset drivers. Currently only ata-ahci, ata-marvell, 
ata-promise and ata-siliconimage relies on this feature. In general, the 
drivers use preallocated DMA workspace to hold small request descriptors, 
consumed by hardware.

All of these drivers do not synchronize workspace with bus_dmamap_sync(). They 
assumes that DMA workspace is coherent. However ATA driver neither enforces 
coherency by using BUS_DMA_COHERENT flag nor synchronizes the workspace with 
bus_dmamap_sync().

I have fixed my problem by adding BUS_DMA_COHERENT flag to workspace 
allocation code:

diff --git a/sys/dev/ata/ata-dma.c b/sys/dev/ata/ata-dma.c
index ab4c3c8..71ee41b 100644
--- a/sys/dev/ata/ata-dma.c
+++ b/sys/dev/ata/ata-dma.c
@@ -95,8 +95,8 @@ ata_dmainit(device_t dev)
                           0, NULL, NULL, &ch->dma.work_tag))
        goto error;

-    if (bus_dmamem_alloc(ch->dma.work_tag, (void **)&ch->dma.work, 0,
-                        &ch->dma.work_map))
+    if (bus_dmamem_alloc(ch->dma.work_tag, (void **)&ch->dma.work,
+                        BUS_DMA_COHERENT, &ch->dma.work_map))
        goto error;

     if (bus_dmamap_load(ch->dma.work_tag, ch->dma.work_map, ch->dma.work,

The question is about multiplatform impact of this change. Especially I am 
curious how everything works without any problems on i386.

-- 
Best Regards.
Piotr Ziecik.