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 X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8BB27C433DB for ; Mon, 22 Mar 2021 10:01:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 556FE61974 for ; Mon, 22 Mar 2021 10:01:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229883AbhCVKA1 (ORCPT ); Mon, 22 Mar 2021 06:00:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47328 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230051AbhCVKAN (ORCPT ); Mon, 22 Mar 2021 06:00:13 -0400 Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [IPv6:2001:4b98:dc2:55:216:3eff:fef7:d647]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2C14AC061762 for ; Mon, 22 Mar 2021 03:00:13 -0700 (PDT) Received: from pendragon.ideasonboard.com (62-78-145-57.bb.dnainternet.fi [62.78.145.57]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id 82CB6AD6; Mon, 22 Mar 2021 11:00:11 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1616407211; bh=dvk/i0Lw7Di7TAe6y1erjsyHKg8/wZVhsKV0TYC49O4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=oy1W47C4++xjz+XqAwjDIubgTz42pYch4KrkbfTCwjdu3uP/l3qyVB3YP8xxC/J3p u+it9YnKxSHui/faVX4jwLJG8+UTS6k4SS98ejh0FyEJKEdt/j9UneQ/BM5BFStly1 PJSgS+6degDT/POK7f3YQ6hMgLvT3AcMyevne1yY= Date: Mon, 22 Mar 2021 11:59:31 +0200 From: Laurent Pinchart To: Hans Verkuil Cc: Tomi Valkeinen , Sakari Ailus , linux-media@vger.kernel.org Subject: Re: vb2_queue type question Message-ID: References: <67107267-69c8-d87d-6579-5e7dac0400fb@xs4all.nl> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <67107267-69c8-d87d-6579-5e7dac0400fb@xs4all.nl> Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org Hi Hans, On Mon, Mar 22, 2021 at 10:49:26AM +0100, Hans Verkuil wrote: > On 22/03/2021 10:18, Tomi Valkeinen wrote: > > Hi Hans, > > > > We were discussing this with Laurent and Sakari, I thought I'd ask if > > you have any feedback on this. > > > > struct vb2_queue has 'type' field, so you can only use a queue for > > buffers of a single type. struct video_device has 'queue' field, so you > > can only use a single queue for a video_device instance. > > > > TI's SoCs have a CSI-2 receiver, with a bunch of DMA engines. The HW > > doesn't care if we are currently capturing pixel buffers or metadata > > buffers (I don't have experience with other HW, but I imagine this > > shouldn't be a rare case). However, due to vb2_queue, the driver needs > > to decide which one to support, which limits the possible use cases. > > > > I was browsing the code, and afaics the type field doesn't do much. It > > is, of course, used to reject queuing buffers of wrong type, and also > > (mostly in mem-2-mem code) to find out if functions are called in input > > or output context. > > > > The latter one could be easily removed by just comparing the given queue > > pointer to a stored pointer (e.g. queue == priv->input_queue). > > > > Do you see any problems if we were to change the type field to > > type_mask, allowing multiple buffer types per queue? Or even remove the > > vb2_queue->type. This raises some questions, like should a queue contain > > only buffers of a single type or can it contain a mix of buffers (I > > think it shouldn't contain a mix of buffers), or can a queue's type_mask > > contain both input and output types (I don't see why not). > > > > An alternate which I tried was creating two vb2_queues, and switching > > the video_device->queue at runtime based on set_format. It kind of > > works, but I think the behavior is a bit unclear, and it might be > > difficult to catch all the corner cases. > > A vb2_queue basically represents a buffer queue that will be fed to a > DMA engine. It assumes that all the buffers are of the same format, > which typically is tied directly to the type. > > The type of a vb2_queue can be changed if you like, but once buffers > are allocated it is fixed and can't be changed again until all buffers > are released. So you can't mix buffers of different types. > > This is actually done in the vivid driver: see vidioc_s_fmt_vbi_cap() > and vidioc_s_fmt_sliced_vbi_cap(): depending on the format the queue > type will be set to either capture raw or sliced VBI. > > The ivtv driver does the same thing. > > So as long as vb2_is_busy() returns false, you are free to change the > queue type. > > There is no need for a type_mask or anything like that. That's up to > the bridge driver to check. The vb2_queue type is there to ensure that > userspace isn't trying to mix buffers of different types, but as long > as no buffers are allocated it doesn't do anything and you are free to > change it. I wasn't aware of this design rationale. It would be useful to expand the documentation of vb2_queue.type to document this. Or have I missed a different location where this is already explained ? -- Regards, Laurent Pinchart