linux-kernel - Re: [PATCH v6 3/4] media: raspberrypi: Add support for RP1-CFE

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Message-ID: <02f05b61-08e7-45f8-8d59-f79bc20d076f@ideasonboard.com>
Date: Mon, 28 Oct 2024 11:21:11 +0200
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Hans Verkuil <hverkuil@...all.nl>
Cc: linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
 linux-arm-kernel@...ts.infradead.org, Naushir Patuck
 <naush@...pberrypi.com>, Laurent Pinchart
 <laurent.pinchart@...asonboard.com>,
 Sakari Ailus <sakari.ailus@...ux.intel.com>,
 Jacopo Mondi <jacopo.mondi@...asonboard.com>,
 Kieran Bingham <kieran.bingham@...asonboard.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>,
 Raspberry Pi Kernel Maintenance <kernel-list@...pberrypi.com>,
 Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>,
 Florian Fainelli <florian.fainelli@...adcom.com>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>
Subject: Re: [PATCH v6 3/4] media: raspberrypi: Add support for RP1-CFE

Hi,

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:
> 
> <snip>
> 
>> +/*
>> + * 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?

  Tomi