From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 16472C83F16 for ; Tue, 29 Aug 2023 11:15:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230436AbjH2LO4 (ORCPT ); Tue, 29 Aug 2023 07:14:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56196 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230219AbjH2LOy (ORCPT ); Tue, 29 Aug 2023 07:14:54 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id C4365BD; Tue, 29 Aug 2023 04:14:50 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 230A32F4; Tue, 29 Aug 2023 04:15:29 -0700 (PDT) Received: from [10.57.3.159] (unknown [10.57.3.159]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A92603F738; Tue, 29 Aug 2023 04:14:46 -0700 (PDT) Message-ID: Date: Tue, 29 Aug 2023 12:14:44 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:102.0) Gecko/20100101 Thunderbird/102.14.0 Subject: Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size Content-Language: en-GB To: Tomasz Figa , Anle Pan , Christoph Hellwig Cc: m.szyprowski@samsung.com, mchehab@kernel.org, linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, hui.fang@nxp.com References: <20230828075420.2009568-1-anle.pan@nxp.com> From: Robin Murphy In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023-08-29 11:03, Tomasz Figa wrote: > Hi Anle, > > On Mon, Aug 28, 2023 at 8:57 AM Anle Pan wrote: >> >> When allocating from pages, the size of the sg segment is unlimited and >> the default is UINT_MAX. This will cause the DMA stream mapping failed >> later with a "swiotlb buffer full" error. > > Thanks for the patch. Good catch. > >> The default maximum mapping >> size is 128 slots x 2K = 256K, determined by "IO_TLB_SEGSIZE". >> To fix the issue, limit the sg segment size according to >> "dma_max_mapping_size" to match the mapping limit. >> >> Signed-off-by: Anle Pan >> --- >> drivers/media/common/videobuf2/videobuf2-dma-sg.c | 9 +++++++-- >> 1 file changed, 7 insertions(+), 2 deletions(-) >> >> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-sg.c b/drivers/media/common/videobuf2/videobuf2-dma-sg.c >> index fa69158a65b1..b608a7c5f240 100644 >> --- a/drivers/media/common/videobuf2/videobuf2-dma-sg.c >> +++ b/drivers/media/common/videobuf2/videobuf2-dma-sg.c >> @@ -105,6 +105,7 @@ static void *vb2_dma_sg_alloc(struct vb2_buffer *vb, struct device *dev, >> struct sg_table *sgt; >> int ret; >> int num_pages; >> + size_t max_segment = 0; >> >> if (WARN_ON(!dev) || WARN_ON(!size)) >> return ERR_PTR(-EINVAL); >> @@ -134,8 +135,12 @@ static void *vb2_dma_sg_alloc(struct vb2_buffer *vb, struct device *dev, >> if (ret) >> goto fail_pages_alloc; >> >> - ret = sg_alloc_table_from_pages(buf->dma_sgt, buf->pages, >> - buf->num_pages, 0, size, GFP_KERNEL); >> + if (dev) dev can't be NULL, see the context above. >> + max_segment = dma_max_mapping_size(dev); >> + if (max_segment == 0) >> + max_segment = UINT_MAX; >> + ret = sg_alloc_table_from_pages_segment(buf->dma_sgt, buf->pages, >> + buf->num_pages, 0, size, max_segment, GFP_KERNEL); > > One thing that I'm not sure about here is that we use > sg_alloc_table_from_pages_segment(), but we actually don't pass the > max segment size (as returned by dma_get_max_seg_size()) to it. > I'm also not exactly sure what's the difference between "max mapping > size" and "max seg size". > +Robin Murphy +Christoph Hellwig I think we could benefit from your > expertise here. dma_get_max_seg_size() represents a capability of the device itself, namely the largest contiguous range it can be programmed to access in a single DMA descriptor/register/whatever. Conversely, dma_max_mapping_size() is a capablity of the DMA API implementation, and represents the largest contiguous mapping it is guaranteed to be able to handle (each segment in the case of dma_map_sg(), or the whole thing for dma_map_page()). Most likely the thing you want here is min_not_zero(max_seg_size, max_mapping_size). > Generally looking at videobuf2-dma-sg, I feel like we would benefit > from some kind of dma_alloc_table_from_pages() that simply takes the > struct dev pointer and does everything necessary. Possibly; this code already looks lifted from drm_prime_pages_to_sg(), and if it's needed here then presumably vb2_dma_sg_get_userptr() also needs it, at the very least. Thanks, Robin.