block: Wording tweaks to write zeroes limits

Improve the documentation of the write zeroes limits, to mention
additional constraints that drivers should observe.  Worth squashing
into commit cf081fca, if that hadn't been pushed already :)

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 7d2b152779df10861591850544f113052899aa3a..7a4a00fdfcce9e62dc93775c571fb352553ca520 100644 (file)
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -331,11 +331,14 @@ typedef struct BlockLimits {
     int64_t discard_alignment;
 
     /* maximum number of bytes that can zeroized at once (since it is
-     * signed, it must be < 2G, if set) */
+     * signed, it must be < 2G, if set), should be multiple of
+     * pwrite_zeroes_alignment. May be 0 if no inherent 32-bit limit */
     int32_t max_pwrite_zeroes;
 
     /* optimal alignment for write zeroes requests in bytes, must be
-     * power of 2, and less than max_pwrite_zeroes if that is set */
+     * power of 2, less than max_pwrite_zeroes if that is set, and
+     * multiple of bs->request_alignment. May be 0 if
+     * bs->request_alignment is good enough */
     uint32_t pwrite_zeroes_alignment;
 
     /* optimal transfer length in bytes (must be power of 2, and