On Wed 20 March 2013 11:06:36 Jon Arne Jørgensen wrote:
> On Mon, Mar 18, 2013 at 09:58:32AM +0100, Bjørn Mork wrote:
> > Hans Verkuil <hverkuil@xxxxxxxxx> writes:
> >
> > >> +/*
> > >> + *
> > >> + * The device delivers data in chunks of 0x400 bytes.
> > >> + * The four first bytes is a magic header to identify the chunks.
> > >> + * 0xaa 0xaa 0x00 0x00 = saa7113 Active Video Data
> > >> + * 0xaa 0xaa 0x00 0x01 = PCM - 24Bit 2 Channel audio data
> > >> + */
> > >> +static void process_packet(struct smi2021_dev *dev, u8 *p, int len)
> > >> +{
> > >> + int i;
> > >> + u32 *header;
> > >> +
> > >> + if (len % 0x400 != 0) {
> > >> + printk_ratelimited(KERN_INFO "smi2021::%s: len: %d\n",
> > >> + __func__, len);
> > >> + return;
> > >> + }
> > >> +
> > >> + for (i = 0; i < len; i += 0x400) {
> > >> + header = (u32 *)(p + i);
> > >> + switch (__cpu_to_be32(*header)) {
> > >
> > > That's not right. You probably mean __be32_to_cpu, that makes more sense.
> > >
> > >> + case 0xaaaa0000: {
> > >> + parse_video(dev, p+i+4, 0x400-4);
> > >> + break;
> > >> + }
> > >> + case 0xaaaa0001: {
> > >> + smi2021_audio(dev, p+i+4, 0x400-4);
> > >> + break;
> > >> + }
> >
> > This could be just me, but I would have done it like this to take
> > advantage of compile time constant conversions (and also dropping the
> > noisy extra {}s):
> >
> > switch (*header) {
> > case cpu_to_be32(0xaaaa0000):
> > parse_video(dev, p+i+4, 0x400-4);
> > break;
> > case cpu_to_be32(0xaaaa0001):
> > smi2021_audio(dev, p+i+4, 0x400-4);
> > break;
> > ..
> >
> >
> > >From the name of the function I assume the difference may actually be
> > measurable here if this runs for every processed packet.
> >
> >
>
> You are both right, I will have a second look at this code.
> I guess I'll try to implement Bjørns suggestion.
>
> As I'm working with a byte-array, I could probably change this code to:
>
> if (header[0] == 0xaa && header[1] == 0xaa
> && header[2] == 0x00 && header[3] == 0x00) {
>
> {...}
>
> } else if (header[0] == 0xaa && header[1] == 0xaa
> && header[2] == 0x00 && header[3] == 0x01) {
>
> {...}
> }
>
> But I hope you agree that the switch statement is cleaner?
>
> (I just find it hard to wrap my head around the big vs. little endian
> differences when dealing with hexadecimal integer notation :) )
Bjørn's solution is best in this case.
Regards,
Hans
>
> > Bjørn
> > --
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