linux-kernel - Re: [RFC 2/4] media: videobuf2: Replace bufs array by a list

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Message-ID: <cbf34cf1-e065-8136-8344-89ca1864f637@xs4all.nl>
Date:   Tue, 14 Mar 2023 11:42:46 +0100
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     David Laight <David.Laight@...LAB.COM>,
        'Laurent Pinchart' <laurent.pinchart@...asonboard.com>,
        Benjamin Gaignard <benjamin.gaignard@...labora.com>
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Subject: Re: [RFC 2/4] media: videobuf2: Replace bufs array by a list

Hi David,

On 3/14/23 11:11, David Laight wrote:
> From: Hans Verkuil
>> Sent: 14 March 2023 08:55
> ...
>> Why not start with a dynamically allocated array of 32 vb2_buffer pointers?
>> And keep doubling the size (reallocing) whenever more buffers are needed,
>> up to some maximum (1024 would be a good initial value for that, I think).
>> This max could be even a module option.
> 
> I don't know the typical uses (or the code at all).
> But it might be worth having a small array in the structure itself.
> Useful if there are typically always (say) less than 8 buffers.
> For larger sizes use the (IIRC) kmalloc_size() to find the actual
> size of the structure that will be allocate and set the array
> size appropriately.

The typical usage is that applications allocate N buffers with the
VIDIOC_REQBUFS ioctl, and in most cases that's all they use. The
current max is 32 buffers, so allocating that initially (usually
during probe()) will cover all current cases with a single one-time
kzalloc.

Only if the application wants to allocate more than 32 buffers will
there be a slight overhead.

Regards,

	Hans

> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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