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

[Krzysztof Opasiak <k.opasiak@samsung.com>]

Hi,

> -----Original Message-----
> From: Michal Nazarewicz [mailto:mpn@google.com] On Behalf Of Michal
> Nazarewicz
> Sent: Monday, June 09, 2014 3:28 PM
> To: Krzysztof Opasiak; 'Felipe Balbi'; Krzysztof Opasiak
> Cc: linux-usb@vger.kernel.org; linux-kernel@vger.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?

Looks very good to me! In my opinion this one should be used, it emphasizes your intentions very well so it's perfect code for sample app.

--
BR's
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics
k.opasiak@samsung.com