Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 6 May 2020 03:55:48 -0300
From:   "Daniel W. S. Almeida" <dwlsalmeida@gmail.com>
To:     Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc:     "=?utf-8?Q?sean=40mess.org?=" <sean@mess.org>,
        "=?utf-8?Q?kstewart=40linuxfoundation.org?=" 
        <kstewart@linuxfoundation.org>,
        "=?utf-8?Q?allison=40lohutok.net?=" <allison@lohutok.net>,
        "=?utf-8?Q?tglx=40linutronix.de?=" <tglx@linutronix.de>,
        "=?utf-8?Q?linux-media=40vger.kernel.org?=" 
        <linux-media@vger.kernel.org>,
        "=?utf-8?Q?skhan=40linuxfoundation.org?=" <skhan@linuxfoundation.org>,
        "=?utf-8?Q?linux-kernel-mentees=40lists.linuxfoundation.org?=" 
        <linux-kernel-mentees@lists.linuxfoundation.org>,
        "=?utf-8?Q?linux-kernel=40vger.kernel.org?=" 
        <linux-kernel@vger.kernel.org>
Message-ID: <40C2F764-6E43-418B-8904-952C5E99D9D9@getmailspring.com>
In-Reply-To: <20200503101621.50047b5c@coco.lan>
References: <20200503101621.50047b5c@coco.lan>
Subject: Re: [RFC, WIP, v4 09/11] media: vidtv: implement a PES
 packetizer
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Mauro,


> As commented, don't use WARN_ON(). At most, you could use WARN_ON_ONCE(),
> as otherwise, you may end by causing serious performance issues if
> the code starts to produce a flood of warnings at the dmesg.
> 
> I would use pr_warn_ratelimit() on all such cases.
> 

OK.


> I don't like the idea of changing the "from" buffer endiannes, copy
> and then restore it back to the original state. Is this really needed?
> 
> I would, instead, define:
> 
> 	struct pes_header {
> 	...
> 		__be32 bitfield;
> 		__be16 length;
> 	...
> 	};
> 
> Then wherever you would touch them:
> 
> 	u32 bitfield;
> 	u16 len;
> 
> 	/* Write into BE fields */
> 	pes_header.bitfield = cpu_to_be32(bitfield);
> 	pes_header.length = cpu_to_be16(len);
> 
> 	/* Read from BE fields */
> 	bitfield = be32_to_cpu(pes_header.bitfield);
> 	len = be16_to_cpu(pes_header.length);
> 
> 
> As a side effect, when you use "__be16" and "__be32" types, gcc
> and smatch/sparse will warn you if you mess with endiannes.
> 
> Same applies to similar code elsewhere.
> 

I don't like it either, it is error prone. I did not know about this
other possibility. Does this work for _bitfields_ though?

I think the authors for libdvbv5 used unions precisely so bswap() could
be called on a 'bitfield' member and from then on the bitfields could be
accessed directly, e.g.:

	union {
		u16 bitfield; <-- call bswap() on this
		struct {
                        --> then use these directly:
			u8  syntax:1;
			u8  zero:1;
			u8  one:2;
			u16 section_length:12;
		} __packed;
	} __packed

At least that's what I understood.

I found this: 
https://lwn.net/Articles/741762/

Maybe *_get_bits, *_replace_bits is the equivalent that I should use for bitfields?

Because I'd rather not do this:

> 	u32 bitfield;
> 	/* Write into BE fields */
> 	pes_header.bitfield = cpu_to_be32(bitfield);

Since I'd have to write the (many!) bitwise operations myself and I'm
sure I will mess this up at _some_ point.


thanks,
- Daniel