On 04/17/2014 03:13 PM, Mauro Carvalho Chehab wrote: > Em Thu, 17 Apr 2014 11:49:51 +0200 > Hans Verkuil <hverkuil@xxxxxxxxx> escreveu: > >> On 04/17/2014 04:17 AM, Mauro Carvalho Chehab wrote: >>> Em Thu, 17 Apr 2014 00:33:55 +0200 >>> Hans Verkuil <hverkuil@xxxxxxxxx> escreveu: >>> >>>> On 04/17/2014 12:23 AM, Mauro Carvalho Chehab wrote: >>>>> Em Mon, 10 Mar 2014 13:20:49 +0100 >>>>> Hans Verkuil <hverkuil@xxxxxxxxx> escreveu: >>>>> >>>>>> From: Hans Verkuil <hans.verkuil@xxxxxxxxx> >>>>>> >>>>>> Convert the saa7134 driver to vb2. >>>>>> >>>>>> Note that while this uses the vb2-dma-sg version, the VB2_USERPTR mode is >>>>>> disabled. The DMA hardware only supports DMAing full pages, and in the >>>>>> USERPTR memory model the first and last scatter-gather buffer is almost >>>>>> never a full page. >>>>>> >>>>>> In practice this means that we can't use the VB2_USERPTR mode. >>>>> >>>>> Why not? Provided that the buffer is equal or bigger than the number of >>>>> pages required by saa7134, that should be OK. >>>>> >>>>> All the driver needs to do is to check if the USERPTR buffer condition is met, >>>>> returning an error otherwise (and likely printing a msg at dmesg). >>>> >>>> Yuck. Well, I'll take a look at this. >>>> >>>> It has in my view the same problem as abusing USERPTR to pass pointers to >>>> physically contiguous memory: yes, it 'supports' USERPTR, but it has additional >>>> requirements which userspace has no way of knowing or detecting. >>>> >>>> It's really not USERPTR at all, it is PAGE_ALIGNED_USERPTR. This was a bit confusing following the previous paragraph. I meant to say that the *saa7134* userptr implementation is not USERPTR at all but PAGE_ALIGNED_USERPTR. A proper USERPTR implementation (like in bttv) can use any malloc()ed pointer as it should, but saa7134 can't as it requires the application to align the pointer to a page boundary, which is non-standard. Regards, Hans >>>> >>>> Quite different. >>> >>> Hmm... If I remember well, mmapped memory (being userptr or not) are always >>> page aligned, at least on systems with MMU. >> >> Not malloc()ed memory. That's what userptr is about. > > Take a look at videobuf_dma_init_user_locked at > drivers/media/v4l2-core/videobuf-dma-sg.c: > > first = (data & PAGE_MASK) >> PAGE_SHIFT; > last = ((data+size-1) & PAGE_MASK) >> PAGE_SHIFT; > dma->offset = data & ~PAGE_MASK; > dma->size = size; > dma->nr_pages = last-first+1; > dma->pages = kmalloc(dma->nr_pages * sizeof(struct page *), GFP_KERNEL); > > The physical memory is always page aligned, even if VM memory isn't. > The offset there is actually used just to subtract the size, at > videobuf_pages_to_sg(). > > So, with VB1, USERPTR works fine, and no special care is needed on > userspace to align the offset. > > Btw, it seems that VB2 also does the same. Take a look at > vb2_dma_sg_get_userptr(). -- 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
