Messages in this thread | | | Date | Mon, 29 Oct 2007 20:30:05 +0300 | From | Cyrill Gorcunov <> | Subject | Re: [PATCH] Clean up scatterlist.h and introduce macros for readability. |
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[Robert P. J. Day - Mon, Oct 29, 2007 at 01:04:02PM -0400] | | Add some macros to <linux/scatterlist.h> to make the bit manipulation | more readable, and expand on some of the documentation. | | This patch incorporates content from Cyrill Gorcunov | <gorcunov@gmail.com> as well. | | Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca> | | --- | | compile-tested with "make defconfig" under i386. (what kernel | subsection would this fall under?) | | | diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h | index 4571231..ae296c0 100644 | --- a/include/linux/scatterlist.h | +++ b/include/linux/scatterlist.h | @@ -1,10 +1,11 @@ | #ifndef _LINUX_SCATTERLIST_H | #define _LINUX_SCATTERLIST_H | | -#include <asm/types.h> | -#include <asm/scatterlist.h> | #include <linux/mm.h> | #include <linux/string.h> | + | +#include <asm/types.h> | +#include <asm/scatterlist.h> | #include <asm/io.h> | | /* | @@ -12,19 +13,36 @@ | * | * Architectures must provide an unsigned long page_link field in the | * scatterlist struct. We use that to place the page pointer AND encode | - * information about the sg table as well. The two lower bits are reserved | - * for this information. | + * information about the sg table as well. The two lower bits (which we | + * will call the "mask bits" for future reference) are reserved for this | + * information. | * | - * If bit 0 is set, then the page_link contains a pointer to the next sg | - * table list. Otherwise the next entry is at sg + 1. | + * If bit 0 is *not* set, then the next sg entry is simply the next one | + * consecutively in memory. If, however, bit 0 *is* set, we must still | + * go to the next consecutive sg entry in memory, but then we must | + * additionally (and immediately) follow that pointer to the next | + * sg entry in the list. | * | * If bit 1 is set, then this sg entry is the last element in a list. | * | * See sg_next(). | - * | */ | + | +/* | + * Define macros for the mask bits and associated operations. | + */ | + | +#define SG_CHAIN 0x1 | +#define SG_LAST 0x2 | +#define SG_MASK 0x3 | + | +#define sg_is_chain(sg) ((sg)->page_link & SG_CHAIN) | +#define sg_is_last(sg) ((sg)->page_link & SG_LAST) | +#define sg_chain_ptr(sg) \ | + ((struct scatterlist *) ((sg)->page_link & ~SG_MASK)) | +#define sg_page(sg) ((struct page *) ((sg)->page_link & ~SG_MASK)) | | -#define SG_MAGIC 0x87654321 | +#define SG_MAGIC 0x87654321 /* only for CONFIG_DEBUG_SG */ | | /** | * sg_assign_page - Assign a given page to an SG entry | @@ -38,17 +56,17 @@ | **/ | static inline void sg_assign_page(struct scatterlist *sg, struct page *page) | { | - unsigned long page_link = sg->page_link & 0x3; | + unsigned long saved_mask_bits = sg->page_link & SG_MASK; | | /* | * In order for the low bit stealing approach to work, pages | * must be aligned at a 32-bit boundary as a minimum. | */ | - BUG_ON((unsigned long) page & 0x03); | + BUG_ON((unsigned long) page & SG_MASK); | #ifdef CONFIG_DEBUG_SG | BUG_ON(sg->sg_magic != SG_MAGIC); | #endif | - sg->page_link = page_link | (unsigned long) page; | + sg->page_link = saved_mask_bits | (unsigned long) page; | } | | /** | @@ -73,7 +91,6 @@ static inline void sg_set_page(struct scatterlist *sg, struct page *page, | sg->length = len; | } | | -#define sg_page(sg) ((struct page *) ((sg)->page_link & ~0x3)) | | /** | * sg_set_buf - Set sg entry to point at given data | @@ -88,15 +105,6 @@ static inline void sg_set_buf(struct scatterlist *sg, const void *buf, | sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf)); | } | | -/* | - * We overload the LSB of the page pointer to indicate whether it's | - * a valid sg entry, or whether it points to the start of a new scatterlist. | - * Those low bits are there for everyone! (thanks mason :-) | - */ | -#define sg_is_chain(sg) ((sg)->page_link & 0x01) | -#define sg_is_last(sg) ((sg)->page_link & 0x02) | -#define sg_chain_ptr(sg) \ | - ((struct scatterlist *) ((sg)->page_link & ~0x03)) | | /** | * sg_next - return the next scatterlist entry in a list | @@ -179,7 +187,7 @@ static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents, | #ifndef ARCH_HAS_SG_CHAIN | BUG(); | #endif | - prv[prv_nents - 1].page_link = (unsigned long) sgl | 0x01; | + prv[prv_nents - 1].page_link = (unsigned long) sgl | SG_CHAIN; | } | | /** | @@ -193,12 +201,12 @@ static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents, | **/ | static inline void sg_mark_end(struct scatterlist *sgl, unsigned int nents) | { | - sgl[nents - 1].page_link = 0x02; | + sgl[nents - 1].page_link = SG_LAST; | } | | static inline void __sg_mark_end(struct scatterlist *sg) | { | - sg->page_link |= 0x02; | + sg->page_link |= SG_LAST; | } | | /** | -- | ======================================================================== | Robert P. J. Day | Linux Consulting, Training and Annoying Kernel Pedantry | Waterloo, Ontario, CANADA | | http://crashcourse.ca | ======================================================================== |
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
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