linux-kernel - Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver

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Date:   Mon, 6 Apr 2020 13:00:26 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <frankc@...dia.com>, <hverkuil@...all.nl>,
        <sakari.ailus@....fi>, <helen.koike@...labora.com>
CC:     <sboyd@...nel.org>, <linux-media@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-clk@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver


On 4/6/20 12:48 PM, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
>
>
> 04.04.2020 04:25, Sowjanya Komatineni пишет:
> ...
>> +static int tegra_channel_capture_frame(struct tegra_vi_channel *chan,
>> +                                    struct tegra_channel_buffer *buf)
>> +{
>> +     int err = 0;
>> +     u32 thresh, value, frame_start, mw_ack_done;
>> +     int bytes_per_line = chan->format.bytesperline;
>> +
>> +     /* program buffer address by using surface 0 */
>> +     vi_csi_write(chan, TEGRA_VI_CSI_SURFACE0_OFFSET_MSB,
>> +                  (u64)buf->addr >> 32);
>> +     vi_csi_write(chan, TEGRA_VI_CSI_SURFACE0_OFFSET_LSB, buf->addr);
>> +     vi_csi_write(chan, TEGRA_VI_CSI_SURFACE0_STRIDE, bytes_per_line);
>> +
>> +     /*
>> +      * Tegra VI block interacts with host1x syncpt for synchronizing
>> +      * programmed condition of capture state and hardware operation.
>> +      * Frame start and Memory write acknowledge syncpts has their own
>> +      * FIFO of depth 2.
>> +      *
>> +      * Syncpoint trigger conditions set through VI_INCR_SYNCPT register
>> +      * are added to HW syncpt FIFO and when the HW triggers, syncpt
>> +      * condition is removed from the FIFO and counter at syncpoint index
>> +      * will be incremented by the hardware and software can wait for
>> +      * counter to reach threshold to synchronize capturing frame with the
>> +      * hardware capture events.
>> +      */
>> +
>> +     /* increase channel syncpoint threshold for FRAME_START */
>> +     thresh = host1x_syncpt_incr_max(chan->frame_start_sp, 1);
>> +
>> +     /* Program FRAME_START trigger condition syncpt request */
>> +     frame_start = VI_CSI_PP_FRAME_START(chan->portno);
>> +     value = VI_CFG_VI_INCR_SYNCPT_COND(frame_start) |
>> +             host1x_syncpt_id(chan->frame_start_sp);
>> +     tegra_vi_write(chan, TEGRA_VI_CFG_VI_INCR_SYNCPT, value);
>> +
>> +     /* increase channel syncpoint threshold for MW_ACK_DONE */
>> +     buf->mw_ack_sp_thresh = host1x_syncpt_incr_max(chan->mw_ack_sp, 1);
>> +
>> +     /* Program MW_ACK_DONE trigger condition syncpt request */
>> +     mw_ack_done = VI_CSI_MW_ACK_DONE(chan->portno);
>> +     value = VI_CFG_VI_INCR_SYNCPT_COND(mw_ack_done) |
>> +             host1x_syncpt_id(chan->mw_ack_sp);
>> +     tegra_vi_write(chan, TEGRA_VI_CFG_VI_INCR_SYNCPT, value);
>> +
>> +     /* enable single shot capture */
>> +     vi_csi_write(chan, TEGRA_VI_CSI_SINGLE_SHOT, SINGLE_SHOT_CAPTURE);
>> +     chan->capture_reqs++;
>> +
>> +     /* wait for syncpt counter to reach frame start event threshold */
>> +     err = host1x_syncpt_wait(chan->frame_start_sp, thresh,
>> +                              TEGRA_VI_SYNCPT_WAIT_TIMEOUT, &value);
> What is the point of waiting for the frame-start? Why just not to wait
> for the frame-end?

Tegra vi supports double buffering where up on receiving frame start 
before HW received frame end and finish writing capture data to memory, 
we can issue next frame as well a head.

Also MW_ACK timeout can happen incase of HDMI2CSI bridges as well when 
hdmi hot plug happens.

For some sensors down the road we may need to skip few frames in case of 
frame start timeout as well which comes later with subsequent patch series.