Hi Mauro, > -----Original Message----- > From: Mauro Carvalho Chehab [mailto:mchehab@xxxxxxxxxx] > Sent: Monday, December 20, 2010 5:23 AM > To: Aguirre, Sergio > Cc: linux-media@xxxxxxxxxxxxxxx; Warren, Christina; Boateng, Akwasi; > Russell King > Subject: Re: [Query][videobuf-dma-sg] Pages in Highmem handling > > Hi Sergio, > > Em 27-08-2010 11:57, Aguirre, Sergio escreveu: > > Hi, > > > > I see that in current videobuf library, for DMA SG code, these functions > fail when > > Detecting a page in Highmem region: > > > > - videobuf_pages_to_sg > > - videobuf_vmalloc_to_sg > > > > Now, what's the real reason to not allow handling of Highmem pages? > > Is it an assumption that _always_ HighMem is not reachable by DMA? > > > > I guess my point is, OMAP platform (and maybe other platforms) can > handle > > Highmem pages without any problem. I commented these validations: > > > > 65 static struct scatterlist *videobuf_vmalloc_to_sg(unsigned char > *virt, > > 66 int nr_pages) > > 67 { > > > > ... > > > > 77 for (i = 0; i < nr_pages; i++, virt += PAGE_SIZE) { > > 78 pg = vmalloc_to_page(virt); > > 79 if (NULL == pg) > > 80 goto err; > > 81 /* BUG_ON(PageHighMem(pg)); */ > > > > ... > > > > 96 static struct scatterlist *videobuf_pages_to_sg(struct page **pages, > > 97 int nr_pages, int > offset) > > 98 { > > > > ... > > > > 109 /* if (PageHighMem(pages[0])) */ > > 110 /* DMA to highmem pages might not work */ > > 111 /* goto highmem; */ > > 112 sg_set_page(&sglist[0], pages[0], PAGE_SIZE - offset, > offset); > > 113 for (i = 1; i < nr_pages; i++) { > > 114 if (NULL == pages[i]) > > 115 goto nopage; > > 116 /* if (PageHighMem(pages[i])) > > 117 goto highmem; */ > > 118 sg_set_page(&sglist[i], pages[i], PAGE_SIZE, 0); > > 119 } > > > > Can somebody shed any light on this? > > Sorry for taking so long to answer you. > Hey, no worries. That's fine. > Basically, videobuf code were written at Linux 2.4 days, to be used by > bttv driver (and later used by cx88 and saa7134). At that time, there > where > a hack for the usage of highmem (I think it was called bigmem or something > like that). > > As I was not maintaining the code on that time, I'm not really sure what > where > the issues, but I suspect that this were an arch-implementation limit > related to DMA transfers at highmem, on that time, due to x86 intrinsic > limits. I'm not sure about the current limits of newer x86 chips, on 32 > and on 64 bits mode, but i think that this limit doesn't exist anymore. > > So, I suspect that just converting it to a call to dma_capable() should > be enough to fix the issue. Ok, sounds reasonable. > > Yet, as videobuf2 is almost ready for merge, maybe the best is to take > some efforts on testing it, and to be sure that it doesn't contain any > arch-specific limits inside its code. So, do you want me to make a patch for this, or is this already taken care on videobuf2? Regards, Sergio > > Cheers, > Mauro -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
