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From:   Tomasz Figa <tfiga@chromium.org>
Date:   Thu, 6 Aug 2020 14:22:13 +0200
Message-ID: <CAAFQd5AWEOr62OrBfRb2HW53omjYfpvN_BAO+eQdRkR9Cixx9w@mail.gmail.com>
Subject: Re: [PATCH v8 05/14] media: rkisp1: add Rockchip ISP1 subdev driver
To:     Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Cc:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Helen Koike <helen.koike@collabora.com>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@lists.infradead.org>,
        linux-devicetree <devicetree@vger.kernel.org>,
        Eddie Cai <eddie.cai.linux@gmail.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        =?UTF-8?Q?Heiko_St=C3=BCbner?= <heiko@sntech.de>,
        Chen Jacob <jacob2.chen@rock-chips.com>,
        Jeffy <jeffy.chen@rock-chips.com>,
        =?UTF-8?B?6ZKf5Lul5bSH?= <zyc@rock-chips.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hans Verkuil <hans.verkuil@cisco.com>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        kernel@collabora.com, Ezequiel Garcia <ezequiel@collabora.com>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        "list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>, Joerg
        Roedel <joro@8bytes.org>," <linux-arm-kernel@lists.infradead.org>,
        Shunqian Zheng <zhengsq@rock-chips.com>,
        Jacob Chen <cc@rock-chips.com>,
        Allon Huang <allon.huang@rock-chips.com>
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On Thu, Aug 6, 2020 at 11:21 AM Dafna Hirschfeld
<dafna.hirschfeld@collabora.com> 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
> >>