linux-kernel - Re: [PATCHv2 5/5] tools: ffs-test: add compatibility code for old kernels

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Date:	Mon, 09 Jun 2014 15:27:58 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Krzysztof Opasiak <k.opasiak@...sung.com>,
	'Felipe Balbi' <balbi@...com>,
	Krzysztof Opasiak <k.opasiak@...sung.com>
Cc:	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 5/5] tools: ffs-test: add compatibility code for old kernels

On Mon, Jun 09 2014, Krzysztof Opasiak wrote:
> As this is an example which will be copy-paste by many users maybe you
> should you struct usb_functionfs_descs_head and struct
> usb_functionfs_descs_head_v2 instead of direct operations using
> hard-coded offsets to make this function more readable?

v3 uses usb_functionfs_descs_head{_v2,} instead of hard-coded offsets.
I also started wondering if it would make sense to go one step further
and define a temporary structure holding the headers.  Something like:

----------- >8 ---------------------------------------------------------
	/* Read v2 header */
	{
		const struct {
			const struct usb_functionfs_descs_head_v2 header;
			const __le32 counts[];
		} __attribute__((packed)) *const in = descriptors;
		const __le32 *counts = in->counts;
		__u32 flags;

		if (le32_to_cpu(in->header.magic) !=
		    FUNCTIONFS_DESCRIPTORS_MAGIC_V2)
			return 0;
		length = le32_to_cpu(in->header.length);
		if (length <= sizeof in->header)
			return 0;
		length -= sizeof in->header;
		flags = le32_to_cpu(in->header.flags);
		if (flags & ~(FUNCTIONFS_HAS_FS_DESC | FUNCTIONFS_HAS_HS_DESC |
			      FUNCTIONFS_HAS_SS_DESC))
			return 0;

#define GET_NEXT_COUNT_IF_FLAG(ret, flg) do {		\
			if (!(flags & (flg)))		\
				break;			\
			if (length < 4)			\
				return 0;		\
			ret = le32_to_cpu(*counts);	\
			length -= 4;			\
			++counts;			\
		} while (0)

		GET_NEXT_COUNT_IF_FLAG(fs_count, FUNCTIONFS_HAS_FS_DESC);
		GET_NEXT_COUNT_IF_FLAG(hs_count, FUNCTIONFS_HAS_HS_DESC);
		GET_NEXT_COUNT_IF_FLAG(count, FUNCTIONFS_HAS_SS_DESC);

		count = fs_count + hs_count;
		if (!count)
			return 0;
		descs_start = descs_end = (const void *)counts;

#undef GET_NEXT_COUNT_IF_FLAG
	}

----------- >8 ---------------------------------------------------------

	/* Allocate legacy descriptors and copy the data. */
	{
		struct {
			struct usb_functionfs_descs_head header;
			__u8 descriptors[];
		} __attribute__((packed)) *out;

		length = sizeof out->header + (descs_end - descs_start);
		out = malloc(length);
		out->header.magic = cpu_to_le32(FUNCTIONFS_DESCRIPTORS_MAGIC);
		out->header.length = cpu_to_le32(length);
		out->header.fs_count = cpu_to_le32(fs_count);
		out->header.hs_count = cpu_to_le32(hs_count);
		memcpy(out->descriptors, descs_start, descs_end - descs_start);
		*legacy = out;
	}
----------- >8 ---------------------------------------------------------

Thoughts?

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +--<mpn@...gle.com>--<xmpp:mina86@...ber.org>--ooO--(_)--Ooo--
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