Grant Grundler wrote:
On Mon, Jul 21, 2008 at 02:51:08PM +0000, Joel Soete wrote:
Hello all,
given this comment:
* Given a virtual address (vba, arg2) and space id, (sid, arg1),
* load the I/O PDIR entry pointed to by pdir_ptr (arg0). Each IO Pdir
* entry consists of 8 bytes as shown below (MSB == bit 0):
...
and also this:
while (byte_cnt > 0) {
/* clear I/O Pdir entry "valid" bit first */
((unsigned char *) pdir_ptr)[7] = 0;
So if I well understand 'Valid' field of a pdir entry is well of 1 bit but
the code cleanup a all byte?
That's just a convenient way to clobber the bit we care about.
The fact that the rest of the pdir remains available is irrelevant
except for debugging (when we might dump IO Pdir to see the history.)
Ok my worry was because other bits of this bytes was related to DMA behaviour of this U2 (Prefetch, Update, Lock, SafeDMA).
Is coding something like:
#define PTE_VALID_BIT_MASK 0xfffffffffffffffeULL
*pdir_ptr &= PTE_VALID_BIT_MASK;
That's a load/modify store of a 64-bit value.
That substantially more instructions than a single byte store.
Agree (but I am looking first to make this driver reliable)
wouldn't do better what comment says it does?
Yes, but the comment is just describing what needs to happen, not exactly how.
The "entry valid" bit just needs to be cleared and it doesn't matter how.
hth
grant
Tx,
J.
PS:
Well a short test seems to help but doesn't fix all issue:
No that doesn't help at all :_(
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