Hi Nate,
I have 1 input regarding your question:
>From: linux-media-owner@xxxxxxxxxxxxxxx [linux-media-owner@xxxxxxxxxxxxxxx] On Behalf Of Dongsoo, Nathaniel Kim [dongsoo.kim@xxxxxxxxx]
>Sent: Tuesday, May 19, 2009 7:53 AM
>To: Shah, Hardik
>Cc: linux-media@xxxxxxxxxxxxxxx; linux-omap@xxxxxxxxxxxxxxx; Jadav, Brijesh R; Hiremath, Vaibhav
>Subject: Re: [PATCH 3/3] OMAP2/3 V4L2 Display Driver
>
>Hello Hardik,
>
>Reviewing your driver, I found something made me curious.
>
>
>On Wed, Apr 22, 2009 at 3:25 PM, Hardik Shah <hardik.shah@xxxxxx> wrote:
<snip>
>> +/* Buffer setup function is called by videobuf layer when REQBUF ioctl is
>> + * called. This is used to setup buffers and return size and count of
>> + * buffers allocated. After the call to this buffer, videobuf layer will
>> + * setup buffer queue depending on the size and count of buffers
>> + */
>> +static int omap_vout_buffer_setup(struct videobuf_queue *q, unsigned int *count,
>> + unsigned int *size)
>> +{
>> + struct omap_vout_device *vout = q->priv_data;
>> + int startindex = 0, i, j;
>> + u32 phy_addr = 0, virt_addr = 0;
>> +
>> + if (!vout)
>> + return -EINVAL;
>> +
>> + if (V4L2_BUF_TYPE_VIDEO_OUTPUT != q->type)
>> + return -EINVAL;
>> +
>> + startindex = (vout->vid == OMAP_VIDEO1) ?
>> + video1_numbuffers : video2_numbuffers;
>> + if (V4L2_MEMORY_MMAP == vout->memory && *count < startindex)
>> + *count = startindex;
>> +
>> + if ((rotation_enabled(vout->rotation)) && *count > 4)
>> + *count = 4;
>> +
>
>
>
>This seems to be weird to me. If user requests multiple buffers more
>than 4, user cannot recognize that the number of buffers requested is
>forced to change into 4. I'm not sure whether this could be serious or
>not, but it is obvious that user can have doubt about why if user have
>no information about the OMAP H/W.
>Is it really necessary to be configured to 4?
>
>
>Cheers,
>
>Nate
>
We did a very similar thing on omap3 camera driver, not exactly by the number of buffers requested, but more about checking if the bytesize of the total requested buffers was superior to the MMU fixed sized page table size capabilities to map that size, then we were limiting the number of buffers accordingly (for keeping the page table size fixed).
According to the v4l2 spec, changing the count value should be valid, and it is the userspace app responsability to check the value again, to confirm how many of the requested buffers are actually available.
Regards,
Sergio--
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