On Thu, Mar 02 2006, linux-os (Dick Johnson) wrote: > > On Thu, 2 Mar 2006, Jens Axboe wrote: > > > On Thu, Mar 02 2006, linux-os (Dick Johnson) wrote: > >> > >> On Thu, 2 Mar 2006, Ju, Seokmann wrote: > >> > >>> Hi, > >>> > >>> In the 'scsi_cmnd' structure, there are two entries holding address > >>> information for data to be transferred. One is 'request_buffer' and the > >>> other one is 'buffer'. > >>> In case of 'use_sg' is non-zero, those entries indicates the address of > >>> the scatter-gather table. > >>> > >>> Is there way to get virtual address (so that the data could be accessed > >>> by the driver) of the actual data in the case of 'use_sg' is non-zero? > >>> > >>> Any comments would be appreciated. > >>> > >>> > >>> Thank you, > >>> > >>> Seokmann > >>> - > >> > >> There is a macro for this purpose. However, for experiments, in > >> the kernel, you can add PAGE_OFFSET (0xC00000000) to get the virtual > >> address. The macro is __va(a), its inverse is __pa(a). > >> > >> Careful. These things can change. > > > > Bzzt no, this wont work if an iommu is involved. It also wont work if > > the page doesn't have a virtual address mapping (think highmem). > > > > -- > > Jens Axboe > > > > Are you going to get DMA-able pages out of high memory? The guy is > doing scatter-table DMA, i.e., linked DMA. I think you need to > build that table with pages from get_dma_page(). If you use A highmem page is a regular page, it just doesn't have a kernel virtual mapping. You can dma to/from these pages just fine. > highmem, somebody can program the iommu right out from under > the DMA engine while a DMA is in progress because the CPU is > not involved and can be executing lots of code that can do lots > of bad things. Nonsense. You can't have multiple iommu mappings or dma transactions against the same highmem page of course, however that is no different than a low mem page. -- Jens Axboe - : send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
