From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752864AbcAUUT3 (ORCPT ); Thu, 21 Jan 2016 15:19:29 -0500 Received: from smarthost03c.mail.zen.net.uk ([212.23.1.22]:48972 "EHLO smarthost03c.mail.zen.net.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752672AbcAUUT0 (ORCPT ); Thu, 21 Jan 2016 15:19:26 -0500 Message-ID: <1453407559.2858.77.camel@linaro.org> Subject: Re: [PATCH] staging: android: ion: Set the length of the DMA sg entries in buffer From: "Jon Medhurst (Tixy)" To: Laura Abbott Cc: Greg Kroah-Hartman , Arve =?ISO-8859-1?Q?Hj=F8nnev=E5g?= , Riley Andrews , Liviu Dudau , devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org, Robin Murphy Date: Thu, 21 Jan 2016 20:19:19 +0000 In-Reply-To: <56A117B6.7050503@redhat.com> References: <1453377467.2858.14.camel@linaro.org> <56A117B6.7050503@redhat.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.9-1+b1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Originating-smarthost03c-IP: [82.69.122.217] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2016-01-21 at 09:39 -0800, Laura Abbott wrote: > On 01/21/2016 03:57 AM, Jon Medhurst (Tixy) wrote: > > From: Liviu Dudau > > > > ion_buffer_create() will allocate a buffer and then create a DMA > > mapping for it, but it forgot to set the length of the page entries. > > > > Signed-off-by: Liviu Dudau > > Signed-off-by: Jon Medhurst > > --- > > drivers/staging/android/ion/ion.c | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c > > index e237e9f..df56021 100644 > > --- a/drivers/staging/android/ion/ion.c > > +++ b/drivers/staging/android/ion/ion.c > > @@ -251,8 +251,10 @@ static struct ion_buffer *ion_buffer_create(struct ion_heap *heap, > > * memory coming from the heaps is ready for dma, ie if it has a > > * cached mapping that mapping has been invalidated > > */ > > - for_each_sg(buffer->sg_table->sgl, sg, buffer->sg_table->nents, i) > > + for_each_sg(buffer->sg_table->sgl, sg, buffer->sg_table->nents, i) { > > sg_dma_address(sg) = sg_phys(sg); > > + sg_dma_len(sg) = sg->length; > > + } > > mutex_lock(&dev->buffer_lock); > > ion_buffer_add(dev, buffer); > > mutex_unlock(&dev->buffer_lock); > > > > So Ion is really doing it wrong by setting the sg_dma_address manually as > the comment above notes. Ion has moved away from sg_dma_len though > (see 06e0dcaeb4fd72a010a1f5ad0c03abd8e0a58ef9). This isn't technically > a mapping as well. What's broken by not having sg_dma_len set? I fear this could end up being embarrassing... What's broken is that the out-of-tree kernel driver for ARM's Mali GPU is getting passed a dma_buf corresponding to the ION buffer. It is then calling dma_buf_map_attachment [1] on that and then parsing the resultant scatter-gather list to get the physical pages so it can pass them to the GPU hardware. In the process, it is using sg_dma_len() to get the length, which is garbage for ION buffers if ion_buffer_create() doesn't set it. [1] http://git.linaro.org/landing-teams/working/arm/kernel-release.git/blob/9660bff61ab296be02aad111d0bc2b9919493de5:/drivers/gpu/arm/midgard/mali_kbase_jd.c#l333 Now, I just tried making the Mali driver use sg->length rather than sg_dma_len() and, unsurprisingly, that also fixes the problem. So, my questions would be... Is it acceptable for a driver getting a dma_buf to parse the scatter-gather list for that by had? If so, should it use ->length or sg_dma_len() to get the length of each element? If sg_dma_len() is correct or acceptable then it seems to me that the ION code should set that length. Especially as the comment in the code implies it's faking a call to map_sg and grepping the kernel tree for real implementations of that functionality seems to show the dma_address getting set. As you can probably tell, I feel I may be on shaky ground. This is because I don't fully understanding the code and suspecting both the ION and GPU code is rather dodgy (and possibly the bits in between :-) -- Tixy