Re: [PATCH v8 05/14] media: rkisp1: add Rockchip ISP1 subdev driver | linux-arm-kernel

quoted

On Thu, Aug 6, 2020 at 11:21 AM Dafna Hirschfeld
[off-list ref] wrote:

Am 05.08.20 um 23:10 schrieb Dafna Hirschfeld:
Hi

On 22.07.20 17:24, Tomasz Figa wrote:
Hi Dafna,

On Sat, Jul 11, 2020 at 01:04:31PM +0200, Dafna Hirschfeld wrote:
Hi Laurent,

On 16.08.19 02:13, Laurent Pinchart wrote:
Hello Helen,

Thank you for the patch.

On Tue, Jul 30, 2019 at 03:42:47PM -0300, Helen Koike wrote:
[snip]
+static void rkisp1_isp_queue_event_sof(struct rkisp1_isp_subdev *isp)
+{
+    struct v4l2_event event = {
+        .type = V4L2_EVENT_FRAME_SYNC,
+        .u.frame_sync.frame_sequence =
+            atomic_inc_return(&isp->frm_sync_seq) - 1,
I would move the increment to the caller, hiding it in this function is
error-prone (and if you look at the caller I'm pointing out one possible
error :-)).

In general usage of frm_sync_seq through the driver seems to be very
race-prone. It's read in various IRQ handling functions, all coming from
the same IRQ, so that part is fine (and wouldn't require an atomic
variable), but when read from the buffer queue handlers I really get a
red light flashing in my head. I'll try to investigate more when
reviewing the next patches.
I see that the only place were 'frame_sequence' is read outside of the irq
handlers is in the capture in 'rkisp1_vb2_buf_queue':

    /*
          * If there's no next buffer assigned, queue this buffer directly
          * as the next buffer, and update the memory interface.
          */
         if (cap->is_streaming && !cap->buf.next &&
             atomic_read(&cap->rkisp1->isp.frame_sequence) == -1) {
                 cap->buf.next = ispbuf;
                 rkisp1_set_next_buf(cap);
         } else {
                 list_add_tail(&ispbuf->queue, &cap->buf.queue);
         }
This "if" condition seems very specific, a case where we already stream but v-start was not yet received.
I think it is possible to remove the test 'atomic_read(&cap->rkisp1->isp.frame_sequence) == -1'
from the above condition so that the next buffer is updated in case it is null not just before the first
v-start signal.
We don't have this special case in the Chrome OS code.

I suppose it would make it possible to resume the capture 1 frame
earlier after a queue underrun, as otherwise the new buffer would be
only programmed after the next frame start interrupt and used for the
next-next frame.  However, it's racy, because programming of the buffer
addresses is not atomic and could end up with the hardware using few
plane addresses from the new buffer and few from the dummy buffer.

Given that and also the fact that a queue underrun is a very special
case, where the system was already having problems catching up, I'd just
remove this special case.

[snip]
+void rkisp1_isp_isr(unsigned int isp_mis, struct rkisp1_device *dev)
+{
+    void __iomem *base = dev->base_addr;
+    unsigned int isp_mis_tmp = 0;
_tmp are never good names :-S

+    unsigned int isp_err = 0;
Neither of these variable need to be initialised to 0.

+
+    /* start edge of v_sync */
+    if (isp_mis & CIF_ISP_V_START) {
+        rkisp1_isp_queue_event_sof(&dev->isp_sdev);
This will increment the frame sequence number. What if the interrupt is
slightly delayed and the next frame starts before we get a change to
copy the sequence number to the buffers (before they will complete
below) ?
Do you mean that we get two sequental v-start signals and then the next
frame-end signal in MI_MIS belongs to the first v-start signal of the two?
How can this be solved? I wonder if any v-start signal has a later signal
that correspond to the same frame so that we can follow it?

Maybe we should have one counter that is incremented on v-start signal,
and another counter that is incremented uppon some other signal?
We're talking about a hard IRQ. I can't imagine the interrupt handler
being delayed for a time close to a full frame interval (~16ms for 60
fps) to trigger such scenario.

+
+        writel(CIF_ISP_V_START, base + CIF_ISP_ICR);
Do you need to clear all interrupt bits individually, can't you write
isp_mis to CIF_ISP_ICR at the beginning of the function to clear them
all in one go ?

+        isp_mis_tmp = readl(base + CIF_ISP_MIS);
+        if (isp_mis_tmp & CIF_ISP_V_START)
+            v4l2_err(&dev->v4l2_dev, "isp icr v_statr err: 0x%x\n",
+                 isp_mis_tmp);
This require some explanation. It looks like a naive way to protect
against something, but I think it could trigger under normal
circumstances if IRQ handling is delayed, and wouldn't do much anyway.
Same for the similar constructs below.

+    }
+
+    if ((isp_mis & CIF_ISP_PIC_SIZE_ERROR)) {
+        /* Clear pic_size_error */
+        writel(CIF_ISP_PIC_SIZE_ERROR, base + CIF_ISP_ICR);
+        isp_err = readl(base + CIF_ISP_ERR);
+        v4l2_err(&dev->v4l2_dev,
+             "CIF_ISP_PIC_SIZE_ERROR (0x%08x)", isp_err);
What does this mean ?

+        writel(isp_err, base + CIF_ISP_ERR_CLR);
+    } else if ((isp_mis & CIF_ISP_DATA_LOSS)) {
Are CIF_ISP_PIC_SIZE_ERROR and CIF_ISP_DATA_LOSS mutually exclusive ?

+        /* Clear data_loss */
+        writel(CIF_ISP_DATA_LOSS, base + CIF_ISP_ICR);
+        v4l2_err(&dev->v4l2_dev, "CIF_ISP_DATA_LOSS\n");
+        writel(CIF_ISP_DATA_LOSS, base + CIF_ISP_ICR);
+    }
+
+    /* sampled input frame is complete */
+    if (isp_mis & CIF_ISP_FRAME_IN) {
+        writel(CIF_ISP_FRAME_IN, base + CIF_ISP_ICR);
+        isp_mis_tmp = readl(base + CIF_ISP_MIS);
+        if (isp_mis_tmp & CIF_ISP_FRAME_IN)
+            v4l2_err(&dev->v4l2_dev, "isp icr frame_in err: 0x%x\n",
+                 isp_mis_tmp);
+    }
+
+    /* frame was completely put out */
"put out" ? :-) What's the difference between ISP_FRAME_IN and ISP_FRAME
? The two comments could do with a bit of brush up, and I think the
ISP_FRAME_IN interrupt could be disabled as it doesn't perform any
action.
Those two oneline comments are just copy-paste from the datasheet.

""
5 MIS_FRAME_IN sampled input frame is complete
1 MIS_FRAME frame was completely put out
""

Unfrotunately, the datasheet does not add any further explanation about those signals.

My loose recollection is that the former is signaled when then frame
is fully input to the ISP and the latter when the ISP completes
outputting the frame to the next block in the pipeline, but someone
would need to verify this, for example by printing timestamps for all
the various interrupts.

+    if (isp_mis & CIF_ISP_FRAME) {
+        u32 isp_ris = 0;
No need to initialise this to 0.

+        /* Clear Frame In (ISP) */
+        writel(CIF_ISP_FRAME, base + CIF_ISP_ICR);
+        isp_mis_tmp = readl(base + CIF_ISP_MIS);
+        if (isp_mis_tmp & CIF_ISP_FRAME)
+            v4l2_err(&dev->v4l2_dev,
+                 "isp icr frame end err: 0x%x\n", isp_mis_tmp);
+
+        isp_ris = readl(base + CIF_ISP_RIS);
+        if (isp_ris & (CIF_ISP_AWB_DONE | CIF_ISP_AFM_FIN |
+                   CIF_ISP_EXP_END | CIF_ISP_HIST_MEASURE_RDY))
+            rkisp1_stats_isr(&dev->stats_vdev, isp_ris);
Is there a guarantee that the statistics will be fully written out
before the video frame itself ? And doesn't this test if any of the
statistics is complete, not all of them ? I think the logic is wrong, it
The datasheet does not add any explanation of what is expected to come first.
Should we wait until all statistics measurements are done? In the struct
sent to userspace there is a bitmaks for which of the statistics are read.
I think that if only part of the statistics are ready, we can already send the once
that are ready to userspace.
If we look further into the code, rkisp1_stats_isr() checks the
interrupt status mask passed to it and reads out only the parameters
with indicated completion. The statistics metadata buffer format
includes a bit mask which tells the userspace which measurements are
available.

However, I think I've spotted a bug there. At the beginning of
rkisp1_stats_isr(), all the 4 interrupt status bits are cleared,
regardless of the mask used later to decide which readouts need to be
done. This could mean that with an unfortunate timing, some measurements
would be lost. So at least the code should be fixed to only clear the
interrupts bits really handled.
I'll fix that
I actually don't think this is a bug. The statistics interrupts are not
enabled and are read from the raw interrupts register. This means
that if we missed a statistics for the current frame and we don't reset it
then we will read it only when the next frame comes out, so it will be
wrongly set as statistics for the next frame although it is actually for the
current frame.
Yes, I noticed that the driver attempts to reduce the number of
interrupts by assuming that the ISP statistics can be read after the
MIS_FRAME interrupt. However, in this case, I don't think we can ever
miss statistics for a frame (unless the system is broken and has
unacceptable interrupt latencies) nor the unfortunate timing I
suggested before could ever take place.

Best regards,
Tomasz

Thanks,
Dafna

As for whether to send separate buffers for each measurement, I guess
it's not a bad thing to let the userspace access the ones available
earlier. Now I only don't recall why we decided to put all the
measurements into one metadata structure, rather than splitting the 4
into their own structures and buffer queues...
Is it possible to have several queues to the same video node?

seems it should be moved out of the CIF_ISP_FRAME test, to a test of its
own. It's hard to tell for sure without extra information though (for
instance why are the stats-related bits read from CIF_ISP_RIS, when
they seem to be documented as valid in CIF_ISP_ISR), but this should be
validated, and most probably fixed. Care should be taken to keep
synchronisation of sequence number between the different queues.
I see that the capture buffers are done before incrementing the frame_sequence with
the following explanation:

    /*
          * Call rkisp1_capture_isr() first to handle the frame that
          * potentially completed using the current frame_sequence number before
          * it is potentially incremented by rkisp1_isp_isr() in the vertical
          * sync.
          */

I think reading the stats/params should also be done before calling rkisp1_capture_isr
for the same reason. (so to match the correct frame_sequence)
My recollection of the sequence of interrupts in this hardware is like
this:

CIF_ISP_V_START (frame 0)
   CIF_ISP_FRAME_IN (frame 0)
     CIF_ISP_FRAME (frame 0)
       CIF_ISP_AWB_DONE
       CIF_ISP_AFM_FIN
       CIF_ISP_EXP_END
       CIF_ISP_HIST_MEASURE_RDY
       CIF_MI_FRAME*
       CIF_ISP_V_START (frame 1)
         CIF_ISP_FRAME_IN (frame 1)
           CIF_ISP_FRAME (frame 1)
             ...

where the interrupts at the same indentation level can happen
independently of each other. Again, someone would have to verify this.
I wrote this patch to print the interrupts and the time difference between interrupts:
https://gitlab.collabora.com/dafna/linux/-/commit/9b9c5ddc2f06a6b87d2c1b210219f69de83296c5

I got this output: http://ix.io/2tl8,
there is a repeating pattern where only v-start interrupt is sent, indicated by the prints "isp mis 0x00000040" then about 23 milisec later are the other interrupts
(FRAME_IN, FRAME, MI_FRAME* ) and about 10 milisec the v-start interrupt again.

I am still not sure why the mi_frame interrupt should be handled first. If it happen for example that all the interrupts arrive at once, how can
we know that the MI_FRAME interrupt relates to the previous v-start interrupt and not the current one?
I think that for that we need a code that keep track of the previous interrupt.

Thanks,
Dafna

Best regards,
Tomasz
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