linux-kernel - Re: [v10 3/4] media: vidtv: add a bridge driver

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Message-ID: <alpine.DEB.2.21.2009151345001.31296@ramsan.of.borg>
Date:   Tue, 15 Sep 2020 13:53:29 +0200 (CEST)
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     "Daniel W. S. Almeida" <dwlsalmeida@...il.com>
cc:     mchehab+huawei@...nel.org, r.verdejo@...sung.com,
        nicolas@...fresne.ca, linux-media@...r.kernel.org,
        skhan@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v10 3/4] media: vidtv: add a bridge driver

 	Hi Daniel,

On Fri, 21 Aug 2020, Daniel W. S. Almeida wrote:
> From: "Daniel W. S. Almeida" <dwlsalmeida@...il.com>
>
> Digital TV devices consist of several independent hardware components
> which are controlled by different drivers.
> Each media device is controlled by a group of cooperating drivers with the
> bridge driver as the main driver.
>
> This patch adds a bridge driver for the Virtual Digital TV driver [vidtv].

This is now commit f90cf6079bf67988 ("media: vidtv: add a bridge
driver") in the media tree.

noreply@...erman.id.au reported the following error for an m68k
allmodconfig build:

     ERROR: modpost: "__udivdi3" [drivers/media/test-drivers/vidtv/dvb-vidtv-bridge.ko] undefined!

Presumably this fails on other 32-bit platforms, too.

> --- /dev/null
> +++ b/drivers/media/test-drivers/vidtv/vidtv_mux.c
> +static u32 vidtv_mux_check_mux_rate(struct vidtv_mux *m)
> +{
> +	/*
> +	 * attempt to maintain a constant mux rate, padding with null packets
> +	 * if needed
> +	 */
> +
> +	u32 nbytes = 0;  /* the number of bytes written by this function */
> +
> +	u64 nbytes_expected; /* the number of bytes we should have written */
> +	u64 nbytes_streamed; /* the number of bytes we actually wrote */
> +	u32 num_null_pkts; /* number of null packets to bridge the gap */
> +
> +	u64 elapsed_time_msecs = jiffies_to_usecs(m->timing.current_jiffies -
> +						  m->timing.past_jiffies);
> +
> +	elapsed_time_msecs = min(elapsed_time_msecs, (u64)VIDTV_MAX_SLEEP_USECS / 1000);
> +	nbytes_expected = div64_u64(m->mux_rate_kbytes_sec * 1000, MSEC_PER_SEC);

Seriously?!?

You multiply by 1000 first, followed by a division by 1000 using an
expensive 64-by-64 division?

> +	nbytes_expected *= elapsed_time_msecs;
> +
> +	nbytes_streamed = m->mux_buf_offset;
> +
> +	if (nbytes_streamed < nbytes_expected) {
> +		/* can't write half a packet: roundup to a 188 multiple */
> +		nbytes_expected  = roundup(nbytes_expected - nbytes_streamed, TS_PACKET_LEN);

drivers/media/test-drivers/vidtv/vidtv_mux.o: In function `vidtv_mux_tick':
vidtv_mux.c:(.text+0x788): undefined reference to `__udivdi3'

This is a 64-by-32 division, hence it should use a helper from
<linux/math64.h>.

However, I'm wondering if "nbytes_expected - nbytes_streamed" is
guaranteed to be a "small" number, hence a 32-by-32 division would be
sufficient?

> +		num_null_pkts    = nbytes_expected / TS_PACKET_LEN;

Likewise.

> +		nbytes          += vidtv_mux_pad_with_nulls(m, num_null_pkts);
> +	}
> +
> +	return nbytes;
> +}

> --- /dev/null
> +++ b/drivers/media/test-drivers/vidtv/vidtv_s302m.c
> +static void vidtv_s302m_compute_pts(struct vidtv_encoder *e)
> +{
> +	u64 count = e->sample_count;
> +	struct vidtv_access_unit *au = e->access_units;
> +
> +	while (au) {
> +		count += au->num_samples;
> +
> +		au->pts = count *
> +			  CLOCK_UNIT_90KHZ / e->sampling_rate_hz;

drivers/media/test-drivers/vidtv/vidtv_s302m.o: In function `vidtv_s302m_encode':
vidtv_s302m.c:(.text+0x2ac): undefined reference to `__udivdi3'

Likewise.

> +
> +		au = au->next;
> +	}
> +}

Gr{oetje,eeting}s,

 						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
 							    -- Linus Torvalds