From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tomasz Stanislawski Subject: Re: [PATCHv10 23/26] v4l: vb2-dma-contig: align buffer size to PAGE_SIZE Date: Fri, 12 Oct 2012 10:24:34 +0200 Message-ID: <5077D3C2.7010207@samsung.com> References: <1349880405-26049-1-git-send-email-t.stanislaws@samsung.com> <1349880405-26049-24-git-send-email-t.stanislaws@samsung.com> <11844448.MZrezqZD1L@avalon> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-reply-to: <11844448.MZrezqZD1L@avalon> Sender: linux-media-owner@vger.kernel.org To: Laurent Pinchart Cc: linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, airlied@redhat.com, m.szyprowski@samsung.com, kyungmin.park@samsung.com, sumit.semwal@ti.com, daeinki@gmail.com, daniel.vetter@ffwll.ch, robdclark@gmail.com, pawel@osciak.com, linaro-mm-sig@lists.linaro.org, hverkuil@xs4all.nl, remi@remlab.net, subashrp@gmail.com, mchehab@redhat.com, zhangfei.gao@gmail.com, s.nawrocki@samsung.com, k.debski@samsung.com List-Id: dri-devel@lists.freedesktop.org Hi Laurent, On 10/11/2012 11:31 PM, Laurent Pinchart wrote: > Hi Tomasz, > > On Wednesday 10 October 2012 16:46:42 Tomasz Stanislawski wrote: >> Most operations on DMA and DMABUF framework need page >> aligned buffers. > > The comment is a bit misleading, the buffer is already page-aligned (unless > I'm mistaken dma_alloc_coherent() returns a page-aligned buffer) but its size > isn't a multiple of the page size. Ok. I will update the commit message that only buffer size is going to be page aligned. > > Do we really need a page size multiple ? Isn't it enough to make the size a > multiple of the cache line size ? > Frankly, I strongly oppose forcing a size of a DMA buffer to be rounded up. However, I discovered a problem while testing mmap() interface in dma-buf. The test in dma_buf_mmap() will fail if the size is not a multiple of 4k. Maybe the value from dma-buf.c:456 should be changed from: dmabuf->size >> PAGE_SHIFT to PAGE_ALIGN(dmabuf->size) >> PAGE_SHIFT However, I preferred to avoid any changes outside of the media tree hoping that the patchset gets merged. Rounding the buffer size to a page size was quick workaround for the issue with DMABUF mmap(). Regards, Tomasz Stanislawski >> This fix guarantees this requirement >> for vb2-dma-contig buffers. >> >> Signed-off-by: Tomasz Stanislawski >> --- >> drivers/media/v4l2-core/videobuf2-dma-contig.c | 3 +++ >> 1 file changed, 3 insertions(+) >> >> diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c >> b/drivers/media/v4l2-core/videobuf2-dma-contig.c index 571a919..002ee50 >> 100644 >> --- a/drivers/media/v4l2-core/videobuf2-dma-contig.c >> +++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c >> @@ -162,6 +162,9 @@ static void *vb2_dc_alloc(void *alloc_ctx, unsigned long >> size) if (!buf) >> return ERR_PTR(-ENOMEM); >> >> + /* align image size to PAGE_SIZE */ >> + size = PAGE_ALIGN(size); >> + >> buf->vaddr = dma_alloc_coherent(dev, size, &buf->dma_addr, GFP_KERNEL); >> if (!buf->vaddr) { >> dev_err(dev, "dma_alloc_coherent of size %ld failed\n", size);