Hi Tomasz,
A few more comments, better late than never I guess.
On Mon, Jun 12, 2017 at 06:59:18PM +0900, Tomasz Figa wrote:
...
> > +/*
> > + * The CSI2 receiver has several parameters affecting
> > + * the receiver timings. These depend on the MIPI bus frequency
> > + * F in Hz (sensor transmitter rate) as follows:
> > + * register value = (A/1e9 + B * UI) / COUNT_ACC
> > + * where
> > + * UI = 1 / (2 * F) in seconds
> > + * COUNT_ACC = counter accuracy in seconds
> > + * For IPU3 COUNT_ACC = 0.0625
> > + *
> > + * A and B are coefficients from the table below,
> > + * depending whether the register minimum or maximum value is
> > + * calculated.
> > + * Minimum Maximum
> > + * Clock lane A B A B
> > + * reg_rx_csi_dly_cnt_termen_clane 0 0 38 0
> > + * reg_rx_csi_dly_cnt_settle_clane 95 -8 300 -16
> > + * Data lanes
> > + * reg_rx_csi_dly_cnt_termen_dlane0 0 0 35
> > + * reg_rx_csi_dly_cnt_settle_dlane0 85 -2 145 -6
> > + * reg_rx_csi_dly_cnt_termen_dlane1 0 0 35 4
> > + * reg_rx_csi_dly_cnt_settle_dlane1 85 -2 145 -6
> > + * reg_rx_csi_dly_cnt_termen_dlane2 0 0 35 4
> > + * reg_rx_csi_dly_cnt_settle_dlane2 85 -2 145 -6
> > + * reg_rx_csi_dly_cnt_termen_dlane3 0 0 35 4
> > + * reg_rx_csi_dly_cnt_settle_dlane3 85 -2 145 -6
> > + *
> > + * We use the minimum values of both A and B.
>
> Why?
>
> > + */
> > +static int cio2_rx_timing(s32 a, s32 b, s64 freq)
> > +{
> > + int r;
> > + const u32 accinv = 16;
> > + const u32 ds = 8; /* divde shift */
>
> typo: divide
>
> > +
> > + freq = (s32)freq >> ds;
>
> Why do we demote freq from 64 to 32 bits here?
I don't think there's any reason to. The original purpose of the check has
likely been to avoid dividing by a 64-bit number but that has been lost
here. The cast should be elsewhere...
>
> > + if (WARN_ON(freq <= 0))
> > + return -EINVAL;
>
> It generally doesn't make sense for the frequency to be negative, so
> maybe the argument should have been unsigned to start with? (And
> 32-bit if we don't expect frequencies higher than 4 GHz anyway.)
The value comes from a 64-bit integer V4L2 control so that implies the value
range of s64 as well.
>
> > +
> > + /* b could be 0, -2 or -8, so r < 500000000 */
>
> Definitely. Anything <= 0 is also less than 500000000. Let's take a
> look at the computation below again:
>
> 1) accinv is multiplied by b,
> 2) 500000000 is divided by 256 (=== shift right by 8 bits) = 1953125,
> 3) accinv*b is multiplied by 1953125 to form the value of r.
>
> Now let's see at possible maximum absolute values for particular steps:
> 1) 16 * -8 = -128 (signed 8 bits),
> 2) 1953125 (unsigned 21 bits),
> 3) -128 * 1953125 = -249999872 (signed 29 bits).
>
> So I think the important thing to note in the comment is:
>
> /* b could be 0, -2 or -8, so |accinv * b| is always less than (1 <<
> ds) and thus |r| < 500000000. */
>
> > + r = accinv * b * (500000000 >> ds);
>
> On the other hand, you lose some precision here. If you used s64
> instead and did the divide shift at the end ((accinv * b * 500000000)
> >> ds), for the example above you would get -250007629. (Depending on
> how big freq is, it might not matter, though.)
>
The frequency is typically hundreds of mega-Hertz.
> Also nit: What is 500000000? We have local constants defined above, I
> think it could also make sense to do the same for this one. The
> compiler should do constant propagation and simplify respective
> calculations anyway.
COUNT_ACC in the formula in the comment a few decalines above is in
nanoseconds. Performing the calculations in integer arithmetics results in
having 500000000 in the resulting formula.
So this is actually a constant related to the hardware but it does not have
a pre-determined name because it is derived from COUNT_ACC.
...
> > +static int cio2_vb2_queue_setup(struct vb2_queue *vq,
> > + unsigned int *num_buffers,
> > + unsigned int *num_planes,
> > + unsigned int sizes[],
> > + struct device *alloc_devs[])
> > +{
> > + struct cio2_device *cio2 = vb2_get_drv_priv(vq);
> > + struct cio2_queue *q = container_of(vq, struct cio2_queue, vbq);
> > + u32 width = q->subdev_fmt.width;
> > + u32 height = q->subdev_fmt.height;
> > + u32 pixelformat = q->pixelformat;
> > + unsigned int i, szimage;
> > + int r = 0;
> > +
> > + for (i = 0; i < ARRAY_SIZE(cio2_csi2_fmts); i++) {
> > + if (pixelformat == cio2_csi2_fmts[i])
> > + break;
> > + }
> > +
> > + /* Use SRGGB10 instead of return err */
> > + if (i >= ARRAY_SIZE(cio2_csi2_fmts))
>
> I think this should be impossible, since S_FMT should have already
> validated (and corrected) the setting.
>
> > + pixelformat = V4L2_PIX_FMT_IPU3_SRGGB10;
> > +
> > + alloc_devs[0] = &cio2->pci_dev->dev;
>
> Hmm, so it doesn't go through the IPU MMU in the end?
No. The CSI-2 receiver isn't behind the MMU --- it's entirely separate from
the ISP.
...
> > +static int cio2_v4l2_querycap(struct file *file, void *fh,
> > + struct v4l2_capability *cap)
> > +{
> > + struct cio2_device *cio2 = video_drvdata(file);
> > +
> > + strlcpy(cap->driver, CIO2_NAME, sizeof(cap->driver));
> > + strlcpy(cap->card, CIO2_DEVICE_NAME, sizeof(cap->card));
> > + snprintf(cap->bus_info, sizeof(cap->bus_info),
> > + "PCI:%s", pci_name(cio2->pci_dev));
> > + cap->device_caps = V4L2_CAP_VIDEO_CAPTURE | V4L2_CAP_STREAMING;
>
> Hmm, I thought single plane queue type was deprecated these days and
> _MPLANE recommended for all new drivers. I'll defer this to other
> reviewers, though.
If the device supports single plane formats only, I don't see a reason to
use MPLANE buffer types.
> [snip]
> > +
> > + /* Initialize vbq */
> > + vbq->type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
> > + vbq->io_modes = VB2_USERPTR | VB2_MMAP;
>
> VB2_DMABUF?
>
> > + vbq->ops = &cio2_vb2_ops;
> > + vbq->mem_ops = &vb2_dma_sg_memops;
> > + vbq->buf_struct_size = sizeof(struct cio2_buffer);
> > + vbq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
> > + vbq->min_buffers_needed = 1;
> > + vbq->drv_priv = cio2;
> > + vbq->lock = &q->lock;
>
> Does the code take into account queue operations and video device
> operations being asynchronous regarding each other? Given that in this
> case there is always one queue per video device, maybe it would just
> make sense to use the same lock for both? (This happens if you leave
> vbq->lock with NULL.)
Using the same lock should be fine IMO.
--
Sakari Ailus
e-mail: sakari.ailus@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx
