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Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1t5iyy-0000000Dwsi-0F7R; Tue, 29 Oct 2024 09:57:48 +0000 Received: from perceval.ideasonboard.com ([213.167.242.64]) by bombadil.infradead.org with esmtps (Exim 4.98 #2 (Red Hat Linux)) id 1t5ivP-0000000Dvqt-3weP; Tue, 29 Oct 2024 09:54:09 +0000 Received: from pendragon.ideasonboard.com (81-175-209-231.bb.dnainternet.fi [81.175.209.231]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id B73E996C; Tue, 29 Oct 2024 10:54:02 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1730195642; bh=bt95k+N19wLHDHIdab75A2vRR4XE/NYH1R8VZJGIB/I=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=JyQOIc8WweN5GQFoX4AmUir2N4NaCJrCqUtgfKpZJ3gM9GkDYVItxNDICKbeeIJrs PDgsnhPedxTXXwhMwR+x50y7whA85D1jy1UnCetV0SAPKSn6PJVQRZqxv4SF0disig b5tgQpPh3MHMpgCzP1gT4fLHbLNjCa9w+J/yVL+w= Date: Tue, 29 Oct 2024 11:53:58 +0200 From: Laurent Pinchart To: Hans Verkuil Cc: Tomi Valkeinen , linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, devicetree@vger.kernel.org, linux-rpi-kernel@lists.infradead.org, linux-arm-kernel@lists.infradead.org, Naushir Patuck , Sakari Ailus , Jacopo Mondi , Kieran Bingham , Mauro Carvalho Chehab , Raspberry Pi Kernel Maintenance , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Florian Fainelli , Broadcom internal kernel review list , Rob Herring , Krzysztof Kozlowski Subject: Re: [PATCH v6 3/4] media: raspberrypi: Add support for RP1-CFE Message-ID: <20241029095358.GE22600@pendragon.ideasonboard.com> References: <02f05b61-08e7-45f8-8d59-f79bc20d076f@ideasonboard.com> <74286a86-51b9-4742-bb0c-583d70b1b0a7@xs4all.nl> <505c502e-b67a-4dca-8420-eb87eae4e170@ideasonboard.com> <59cf95be-fb53-4a94-bc6e-f9dca322749d@xs4all.nl> <5832a2f9-c908-4f5a-a3ee-9cb7d23ddab4@ideasonboard.com> <563347aa-4155-47e1-b71a-0107aed83eb6@xs4all.nl> <20241028151713.GI24052@pendragon.ideasonboard.com> <62073d7a-0a4b-4440-90e5-dcce0dec72d7@ideasonboard.com> <20241028163243.GB26852@pendragon.ideasonboard.com> <3f48495c-77bb-4f26-b51f-8b2d44a4ec96@xs4all.nl> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3f48495c-77bb-4f26-b51f-8b2d44a4ec96@xs4all.nl> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20241029_025408_293358_700819AF X-CRM114-Status: GOOD ( 69.23 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Oct 29, 2024 at 09:23:35AM +0100, Hans Verkuil wrote: > On 28/10/2024 17:32, Laurent Pinchart wrote: > > On Mon, Oct 28, 2024 at 05:32:27PM +0200, Tomi Valkeinen wrote: > >> On 28/10/2024 17:17, Laurent Pinchart wrote: > >>> On Mon, Oct 28, 2024 at 12:30:45PM +0100, Hans Verkuil wrote: > >>>> On 28/10/2024 12:25, Tomi Valkeinen wrote: > >>>>> On 28/10/2024 13:13, Hans Verkuil wrote: > >>>>>> On 28/10/2024 12:05, Tomi Valkeinen wrote: > >>>>>>> On 28/10/2024 12:11, Hans Verkuil wrote: > >>>>>>>> On 28/10/2024 10:21, Tomi Valkeinen wrote: > >>>>>>>>> On 24/10/2024 11:20, Hans Verkuil wrote: > >>>>>>>>>> Hi Tomi, > >>>>>>>>>> > >>>>>>>>>> I know this driver is already merged, but while checking for drivers that use > >>>>>>>>>> q->max_num_buffers I stumbled on this cfe code: > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>>> +/* > >>>>>>>>>>> + * vb2 ops > >>>>>>>>>>> + */ > >>>>>>>>>>> + > >>>>>>>>>>> +static int cfe_queue_setup(struct vb2_queue *vq, unsigned int *nbuffers, > >>>>>>>>>>> +               unsigned int *nplanes, unsigned int sizes[], > >>>>>>>>>>> +               struct device *alloc_devs[]) > >>>>>>>>>>> +{ > >>>>>>>>>>> +    struct cfe_node *node = vb2_get_drv_priv(vq); > >>>>>>>>>>> +    struct cfe_device *cfe = node->cfe; > >>>>>>>>>>> +    unsigned int size = is_image_node(node) ? > >>>>>>>>>>> +                    node->vid_fmt.fmt.pix.sizeimage : > >>>>>>>>>>> +                    node->meta_fmt.fmt.meta.buffersize; > >>>>>>>>>>> + > >>>>>>>>>>> +    cfe_dbg(cfe, "%s: [%s] type:%u\n", __func__, node_desc[node->id].name, > >>>>>>>>>>> +        node->buffer_queue.type); > >>>>>>>>>>> + > >>>>>>>>>>> +    if (vq->max_num_buffers + *nbuffers < 3) > >>>>>>>>>>> +        *nbuffers = 3 - vq->max_num_buffers; > >>>>>>>>>> > >>>>>>>>>> This makes no sense: max_num_buffers is 32, unless explicitly set when vb2_queue_init > >>>>>>>>>> is called. So 32 + *nbuffers is never < 3. > >>>>>>>>>> > >>>>>>>>>> If the idea is that at least 3 buffers should be allocated by REQBUFS, then set > >>>>>>>>>> q->min_reqbufs_allocation = 3; before calling vb2_queue_init and vb2 will handle this > >>>>>>>>>> for you. > >>>>>>>>>> > >>>>>>>>>> Drivers shouldn't modify *nbuffers, except in very rare circumstances, especially > >>>>>>>>>> since the code is almost always wrong. > >>>>>>>>> > >>>>>>>>> Looking at this, the original code in the old BSP tree was, which somehow, along the long way, got turned into the above: > >>>>>>>>> > >>>>>>>>> if (vq->num_buffers + *nbuffers < 3) > >>>>>>>>>           *nbuffers = 3 - vq->num_buffers; > >>>>>>>>> > >>>>>>>>> So... I think that is the same as "q->min_reqbufs_allocation = 3"? > >>>>>>>>> > >>>>>>>>> The distinction between min_queued_buffers and > >>>>>>>>> min_reqbufs_allocation, or rather the need for the latter, still > >>>>>>>>> escapes me. If the HW/SW requires N buffers to be queued, why > >>>>>>>>> would we require allocating more than N buffers? > >>>>>>>> > >>>>>>>> min_queued_buffers is easiest to explain: that represents the requirements of the DMA > >>>>>>>> engine, i.e. how many buffers much be queued before the DMA engine can be started. > >>>>>>>> Typically it is 0, 1 or 2. > >>> > >>> That's partly true only. Even if the hardware requires 2 buffers, a > >>> driver can allocate scratch buffers to lower the requirement for > >>> userspace. Setting min_queued_buffers to 1 is usually fine, as there are > >>> few use cases for userspace to start the hardware before a buffer is > >>> available to capture a frame to. A value of 2 is much more problematic, > >>> as it prevents operating with a single buffer. I know using a single > >>> buffer results in frame drops, but there are resource-constrained > >>> systems where application don't always need all the frames (such as the > >>> Raspberry Pi Zero for instance). I very strongly encourage drivers to > >>> never set a min_queued_buffers value higher than 1. > >>> > >>>>>>>> > >>>>>>>> min_reqbufs_allocation is the minimum number of buffers that will be allocated when > >>>>>>>> calling VIDIOC_REQBUFS in order for userspace to be able to stream without blocking > >>>>>>>> or dropping frames. > >>>>>>>> > >>>>>>>> Typically this is 3 for video capture: one buffer is being DMAed, another is queued up > >>>>>>>> and the third is being processed by userspace. But sometimes drivers have other > >>>>>>>> requirements. > >>> > >>> This is exactly why I dislike min_reqbufs_allocation when set based on > >>> this logic, it encodes assumption on userspace use cases that a capture > >>> driver really shouldn't make. > >>> > >>>>>>>> > >>>>>>>> The reason is that some applications will just call VIDIOC_REQBUFS with count=1 and > >>>>>>>> expect it to be rounded up to whatever makes sense. See the VIDIOC_REQBUFS doc in > >>>>>>>> https://hverkuil.home.xs4all.nl/spec/userspace-api/v4l/vidioc-reqbufs.html > >>>>>>>> > >>>>>>>> "It can be smaller than the number requested, even zero, when the driver runs out of > >>>>>>>>    free memory. A larger number is also possible when the driver requires more buffers > >>>>>>>>    to function correctly." > >>>>>>>> > >>>>>>>> How drivers implement this is a mess, and usually the code in the driver is wrong as > >>>>>>>> well. In particular they often did not take VIDIOC_CREATE_BUFS into account, i.e. > >>>>>>>> instead of 'if (vq->num_buffers + *nbuffers < 3)' they would do 'if (*nbuffers < 3)'. > >>>>>>> > >>>>>>> Thanks, this was educational! > >>>>>>> > >>>>>>> So. If I have a driver that has min_queued_buffers = 1, I can use > >>>>>>> VIDIOC_CREATE_BUFS to allocate a single buffer, and then capture > >>>>>>> just one buffer, right? Whereas VIDIOC_REQBUFS would give me > >>>>>>> (probably) three (or two, if the driver does not set > >>>>>>> min_reqbufs_allocation). Three buffers makes sense for full > >>>>>>> streaming, of course. > >>>>>>> > >>>>>>>> When we worked on the support for more than 32 buffers we added min_reqbufs_allocation > >>>>>>>> to let the core take care of this. In addition, this only applies to VIDIOC_REQBUFS, > >>> > >>> I agree it's better to handle it in the core than in drivers, even if I > >>> dislike the feature in the first place. > >>> > >>>>>>>> if you want full control over the number of allocated buffers, then use VIDIOC_CREATE_BUFS, > >>>>>>>> with this ioctl the number of buffers will never be more than requested, although it > >>>>>>>> may be less if you run out of memory. > >>> > >>> On a side note, we should transition libcamera to use VIDIOC_CREATE_BUFS > >>> unconditionally. > >>> > >>>>>>>> > >>>>>>>> I really should go through all existing drivers and fix them up if they try to > >>>>>>>> handle this in the queue_setup function, I suspect a lot of them are quite messy. > >>>>>>>> > >>>>>>>> One thing that is missing in the V4L2 uAPI is a way to report the minimum number of > >>>>>>>> buffers that need to be allocated, i.e. min_queued_buffers + 1. Since if you want > >>>>>>> > >>>>>>> Hmm, so what I wrote above is not correct? One needs min_queued_buffers + 1? Why is that? > >>>>>> > >>>>>> The DMA engine always uses min_queued_buffers, so if there are only that many buffers, > >>>>>> then it can never return a buffer to userspace! So you need one more. That's the absolute > >>>>>> minimum. For smooth capture you need two more to allow time for userspace to process the > >>>>>> buffer. > >>>>> > >>>>> Hmm, ok, I see. Well, I guess my "I want to capture just a single frame" is not a very common case. > >>> > >>> It's not that uncommon, see above. > >>> > >>>>> > >>>>> Can I queue one buffer, start streaming, stop streaming, and get the > >>>>> filled buffer? But then I guess I don't when the buffer has been > >>>>> filled, i.e. when to call stop streaming. > >>>> > >>>> Exactly. If you really want that, then the driver has to be adapted in the way that Laurent > >>>> suggested, i.e. with one or more scratch buffers. But that is not always possible, esp. with > >>>> older hardware without an IOMMU. > >>> > >>> Drivers can always allocate a full-frame scratch buffer in the worst > >>> case. That can waste memory though, which is less than ideal. > >>> > >>>>> So, never mind, I don't actually have any use case for this, just wondering. > >>>>> > >>>>>>> > >>>>>>>> to use CREATE_BUFS you need that information so you know that you have to create > >>>>>>>> at least that number of buffers. We have the V4L2_CID_MIN_BUFFERS_FOR_CAPTURE control, > >>>>>>>> but it is effectively codec specific. This probably should be clarified. > >>>>>>>> > >>>>>>>> I wonder if it wouldn't be better to add a min_num_buffers field to > >>>>>>>> struct v4l2_create_buffers and set it to min_queued_buffers + 1. > >>> > >>> Don't add the +1. We should give userspace the information it needs to > >>> make informed decisions, not make decisions on its behalf. > >>> > >>>>>>> > >>>>>>> I think this makes sense (although I still don't get the +1). > >>>>>>> > >>>>>>> However, based on the experiences from adding the streams features > >>>>>>> to various ioctls, let's be very careful =). The new > >>>>>>> 'min_num_buffers' can be filled with garbage by the userspace. If > >>>>>>> we define the 'min_num_buffers' field to be always filled by the > >>>>>>> kernel, and any value provided from the userspace to be ignored, I > >>>>>>> think it should work. > >>>>>> > >>>>>> I've posted an RFC for this. > >>>>> > >>>>> Thanks, I'll check it out. > >>>>> > >>>>> For the original issue in this thread, I think the correct fix is to > >>>>> remove the lines from cfe_queue_setup(), and add > >>>>> "q->min_reqbufs_allocation = 3". > >>> > >>> Or just don't set min_reqbufs_allocation ? This is a new driver, and it > >>> requires a device-specific userspace to operate the ISP. I don't think > >>> we need to care about applications blindly calling VIDIOC_REQBUFS(1) and > >>> expecting to get more buffers. > >> > >> It doesn't require a device-specific userspace for plain CSI-2 capture. > >> > >> If I understood right, the expected behavior for VIDIOC_REQBUFS is to > >> return enough buffers for "smooth streaming". So even if device-specific > >> userspace would be required, doesn't it still make sense to have > >> min_reqbufs_allocation = 3? > > > > "Smooth streaming" is use case-dependent, you will need different number > > of buffers for different use cases. That's why I don't like hardcoding > > this in a video capture driver. I'd rather expose information about the > > driver behaviour (in particular, how many buffers it will hold on > > without returning anything to userspace until a new buffer gets queued) > > and let applications make a decision. I don't expect applications > > relying on VIDIOC_REQBUFS(1) to work out-of-the-box on Pi 5 anyway, as > > the media graph needs to be configured. > > > >> Or is your point that even a device-specific userspace, which knows > >> exactly what it's doing, would use VIDIOC_REQBUFS, instead of > >> VIDIOC_CREATE_BUFS? > > > > I expect a device-specific userspace not to require drivers to make > > policy decisions on its behalf. > > Remember that libcamera is a specialized library that indeed wants to > make policy decisions itself. But many other drivers for much simpler > pipelines (typically for video receivers) don't need this and can use > the standard V4L2 API. > > My goal is to have the standard V4L2 API behave in a well-defined manner, > while giving enough information to specialized userspace code like libcamera > to do their own thing. I think we all agree on well-defined :-) Where we may not agree is that, regardless of whether an application goes through libcamera, interfaces with V4L2 through another framework, or directly, I don't think the kernel should make policy decisions on behalf of userspace. I'm fine keeping VIDIOC_REQBUFS(1) operating as it was meant to because we have to ensure backward compatibility, but I don't think it's the kind of API we should design today. Let's start by making sure we expose the information userspace needs, and then we can discuss the next step. > >> Also, if I don't set min_reqbufs_allocation, VIDIOC_REQBUFS(1) would > >> still allocate two buffers, not one. -- Regards, Laurent Pinchart