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

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Message-ID: <EA89ED5D-E477-4464-B523-D729796CE0D0@getmailspring.com>
Date:   Tue, 15 Sep 2020 10:26:17 -0300
From:   "Daniel W. S. Almeida" <dwlsalmeida@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     "mchehab+huawei@...nel.org" <mchehab+huawei@...nel.org>,
        "r.verdejo@...sung.com" <r.verdejo@...sung.com>,
        "nicolas@...fresne.ca" <nicolas@...fresne.ca>,
        "linux-media@...r.kernel.org" 
        <linux-media@...r.kernel.org>,
        "skhan@...uxfoundation.org" <skhan@...uxfoundation.org>,
        "linux-kernel-mentees@...ts.linuxfoundation.org" 
        <linux-kernel-mentees@...ts.linuxfoundation.org>,
        "linux-kernel@...r.kernel.org" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [v10 3/4] media: vidtv: add a bridge driver

Hi Geert,

Thanks for bringing this to my attention.


>> +	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?

This entire function is broken and needs a do-over :)

> using an expensive 64-by-64 division?

I am new to kernel development. I wasn't even aware that this was
expensive, to be honest.


>> +	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?

I think so.

I will send a patch to address the things you pointed out in this email.

-- thanks
-- Daniel