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From: "Daniel W. S. Almeida" <dwlsalmeida@gmail.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "mchehab+huawei@kernel.org" <mchehab+huawei@kernel.org>,
	"r.verdejo@samsung.com" <r.verdejo@samsung.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"nicolas@ndufresne.ca" <nicolas@ndufresne.ca>,
	"linux-kernel-mentees@lists.linuxfoundation.org"
	<linux-kernel-mentees@lists.linuxfoundation.org>,
	"linux-media@vger.kernel.org" <linux-media@vger.kernel.org>
Subject: Re: [Linux-kernel-mentees] [v10 3/4] media: vidtv: add a bridge driver
Date: Tue, 15 Sep 2020 10:26:17 -0300	[thread overview]
Message-ID: <EA89ED5D-E477-4464-B523-D729796CE0D0@getmailspring.com> (raw)
In-Reply-To: <alpine.DEB.2.21.2009151345001.31296@ramsan.of.borg>

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
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  reply	other threads:[~2020-09-15 13:26 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-21 12:58 [Linux-kernel-mentees] [v10 0/4] media: vidtv: Implement a virtual DVB driver Daniel W. S. Almeida
2020-08-21 12:58 ` [Linux-kernel-mentees] [v10 1/4] media: vidtv: implement a tuner driver Daniel W. S. Almeida
2020-08-21 12:58 ` [Linux-kernel-mentees] [v10 2/4] media: vidtv: implement a demodulator driver Daniel W. S. Almeida
2020-08-21 12:58 ` [Linux-kernel-mentees] [v10 3/4] media: vidtv: add a bridge driver Daniel W. S. Almeida
2020-09-15 11:53   ` Geert Uytterhoeven
2020-09-15 13:26     ` Daniel W. S. Almeida [this message]
2020-09-15 13:35       ` Geert Uytterhoeven
2020-09-15 18:13         ` Daniel W. S. Almeida
2020-09-16  7:01         ` Mauro Carvalho Chehab
2020-08-21 12:58 ` [Linux-kernel-mentees] [v10 4/4] media: Documentation: vidtv: Add ReST documentation for vidtv Daniel W. S. Almeida
2020-09-11  8:02 ` [Linux-kernel-mentees] [v10 0/4] media: vidtv: Implement a virtual DVB driver Mauro Carvalho Chehab
2020-09-11 12:18   ` Daniel W. S. Almeida
2020-09-11 13:10     ` Mauro Carvalho Chehab
2020-09-11 13:56       ` Mauro Carvalho Chehab
2020-09-12  2:54         ` Daniel W. S. Almeida
2020-09-12  7:38           ` Mauro Carvalho Chehab
2020-09-12  8:21 ` Hans Verkuil
2020-09-12  9:15   ` Mauro Carvalho Chehab
2020-09-12 14:49     ` Daniel W. S. Almeida
2020-09-12 17:57       ` Mauro Carvalho Chehab
2020-09-14  8:33         ` Hans Verkuil
2020-09-12  8:35 ` Hans Verkuil

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