Generic ATA driver DMA coherency issues
jhb at freebsd.org
Wed May 27 18:22:32 UTC 2009
On Wednesday 27 May 2009 6:53:06 am Piotr Zięcik wrote:
> 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
> The ATA generic driver allocates 64k DMA workspace memory which can be
> optionally used by ATA chipset drivers. Currently only ata-ahci,
> 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().
> assumes that DMA workspace is coherent. However ATA driver neither enforces
> coherency by using BUS_DMA_COHERENT flag nor synchronizes the workspace with
> 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.
The flag has no effect on x86.
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