>-----Original Message----- >From: dri-devel <dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of >Marek Szyprowski >Sent: Sunday, March 29, 2020 5:56 AM >To: Ruhl, Michael J <michael.j.ruhl@xxxxxxxxx>; dri- >devel@xxxxxxxxxxxxxxxxxxxxx; linux-samsung-soc@xxxxxxxxxxxxxxx; linux- >kernel@xxxxxxxxxxxxxxx >Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>; David Airlie ><airlied@xxxxxxxx>; Shane Francis <bigbeeshane@xxxxxxxxx>; >stable@xxxxxxxxxxxxxxx; Thomas Zimmermann <tzimmermann@xxxxxxx>; >Alex Deucher <alexander.deucher@xxxxxxx> >Subject: Re: [PATCH v2] drm/prime: fix extracting of the DMA addresses from >a scatterlist > >Hi Michael, > <snip> >> Is there an example of what the scatterlist would look like in this case? > >DMA framework or IOMMU is allowed to join consecutive chunks while >mapping if such operation is supported by the hw. Here is the example: > >Lets assume that we have a scatterlist with 4 4KiB pages of the physical >addresses: 0x12000000, 0x13011000, 0x13012000, 0x11011000. The total >size of the buffer is 16KiB. After mapping this scatterlist to a device >behind an IOMMU it may end up as a contiguous buffer in the DMA (IOVA) >address space. at 0xf0010000. The scatterlist will look like this: > >sg[0].page = 0x12000000 >sg[0].len = 4096 >sg[0].dma_addr = 0xf0010000 >sg[0].dma_len = 16384 >sg[1].page = 0x13011000 >sg[1].len = 4096 >sg[1].dma_addr = 0 >sg[1].dma_len = 0 >sg[2].page = 0x13012000 >sg[2].len = 4096 >sg[2].dma_addr = 0 >sg[2].dma_len = 0 >sg[3].page = 0x11011000 >sg[3].len = 4096 >sg[3].dma_addr = 0 >sg[3].dma_len = 0 > >(I've intentionally wrote page as physical address to make it easier to >understand, in real SGs it is stored a struct page pointer). > >> Does each SG entry always have the page and dma info? or could you have >> entries that have page information only, and entries that have dma info >only? >When SG is not mapped yet it contains only the ->pages and ->len >entries. I'm not aware of the SGs with the DMA information only, but in >theory it might be possible to have such. >> If the same entry has different size info (page_len = PAGE_SIZE, >> dma_len = 4 * PAGE_SIZE?), are we guaranteed that the arrays (page and >addrs) have >> been sized correctly? > >There are always no more DMA related entries than the phys pages. If >there is 1:1 mapping between physical memory and DMA (IOVA) space, then >each SG entry will have len == dma_len, and dma_addr will be describing >the same as page entry. DMA mapping framework is allowed only to join >entries while mapping to DMA (IOVA). > >> Just trying to get my head wrapped around this. > >Sure, I hope my explanation helps a bit. That is a great example! Thank you very much for the explanation. I was somehow seeing it as the dma side getting split and extended (rather than consolidated) into more possible entries. This clarifies the issue for me. Thanks! Mike >Best regards >-- >Marek Szyprowski, PhD >Samsung R&D Institute Poland > >_______________________________________________ >dri-devel mailing list >dri-devel@xxxxxxxxxxxxxxxxxxxxx >https://lists.freedesktop.org/mailman/listinfo/dri-devel
