linux-kernel - Re: [PATCH V2] w1: ds2408: add magic sequence to disable P0 test mode

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Message-Id: <1A322F9E-F2A7-42E6-8C37-8A5C3F2E28FD@gmail.com>
Date:	Thu, 9 May 2013 15:33:23 -0400
From:	Jean-François Dagenais <jeff.dagenais@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	zbr@...emap.net, linux-kernel@...r.kernel.org,
	Greg KH <greg@...ah.com>
Subject: Re: [PATCH V2] w1: ds2408: add magic sequence to disable P0 test mode

On 2013-05-09, at 2:03 PM, Andrew Morton wrote:
[...]
>> +static int w1_f29_disable_test_mode(struct w1_slave *sl)
>> +{
>> +	int res;
>> +	u8 magic[10] = {0x96, };
>> +	u64 rn = le64_to_cpu(*((u64*)&sl->reg_num));
>> +	memcpy(&magic[1], &rn, 8);
>> +	magic[9] = 0x3C;
> 
> (please put a blank line between end-of-locals and start-of-code)
> 
> The casting looks decidedly dodgy.  I guess it won't cause an unalignned
> exception due to reg_num's alignment, but it appears to defeat the
> endianness handling in the definotion of `struct w1_reg_num'.
> 
> Are you sure this will work OK with all architectures and both
> __LITTLE_ENDIAN_BITFIELD and __BIG_ENDIAN_BITFIELD?

To be honest, I didn't really thought about it that much, I just copy pasted that from Evgeniy Polyakov's hunk at drivers/w1/w1_io.c, function w1_reset_select_slave(struct w1_slave *sl) exept I changed the MATCH_ROM with magic 0x96 and appended magic 0x3C. I have tested it only on the available platform I have which is x86. I agree it looks dodgy. Do you have an alternative? You are certainly more familiar with the kernel's fancy bit and endian tools than I am. I'd be willing to test prior to sending V3.

struct w1_reg_num
{
#if defined(__LITTLE_ENDIAN_BITFIELD)
	__u64	family:8,
		id:48,
		crc:8;
#elif defined(__BIG_ENDIAN_BITFIELD)
	__u64	crc:8,
		id:48,
		family:8;
#else
#error "Please fix <asm/byteorder.h>"
#endif
};

On the wire, the family byte should be sent first, then the MSB of id, then the rest of id and finally the crc.

Perhaps Evgeniy can chime in here?

Cheers,
/jfd--
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