Messages in this thread | | | Date | Fri, 17 May 2002 21:24:17 +0200 | From | Gunther Mayer <> | Subject | Re: [PATCH] IDE PIO write Fix #2 |
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Martin Dalecki wrote:
> U¿ytkownik Linus Torvalds napisa³: > > In article <3CE0D6DE.8090407@evision-ventures.com>, > > Martin Dalecki <dalecki@evision-ventures.com> wrote: > > > >>>--- linux-2.5.15/drivers/ide/ide-taskfile.c.orig Fri May 10 11:49:35 2002 > >>>+++ linux-2.5.15/drivers/ide/ide-taskfile.c Tue May 14 10:40:43 2002 > >>>@@ -606,7 +606,7 @@ > >>> if (!ide_end_request(drive, rq, 1)) > >>> return ide_stopped; > >>> > >>>- if ((rq->current_nr_sectors==1) ^ (stat & DRQ_STAT)) { > >>>+ if ((rq->nr_sectors == 1) ^ ((stat & DRQ_STAT) != 0)) { > >> > > > > Well, that's definitely an improvement - the original code makes no > > sense at all, since it's doing a bitwise xor on two bits that are not > > the same, and then uses that as a boolean value. > > > > Your change at least makes it use the bitwise xor on properly logical > > values, making the bitwise xor work as a _logical_ xor. > > > > Although at that point I'd just get rid of the xor, and replace it by > > the "!=" operation - which is equivalent on logical ops. > > > > > >>> pBuf = ide_map_rq(rq, &flags); > >>> DTF("write: %p, rq->current_nr_sectors: %d\n", pBuf, (int) rq->current_nr_sectors); > >> > >> > >>Hmm. There is something else that smells in the above, since the XOR operator > >>doesn't seem to be proper. Why shouldn't we get DRQ_STAT at all on short > >>request? Could you perhaps just try to replace it with an OR? > > > > > > The XOR operation is a valid op, if you just use it on valid values, > > which the patch does seem to make it do. > > > > I don't know whether the logic is _correct_ after that, but at least > > there is some remote chance that it might make sense. > > > > Linus > > As far as I can see the patch makes sense. It is just exposing a problem > which was hidden before.
The original code: a) if ((rq->current_nr_sectors==1) ^ (stat & DRQ_STAT)) { is being compared to these expressions: b) logical equ. if ((rq->current_nr_sectors==1) || ((stat & DRQ_STAT)!=0)) { (me) c) not eqival. if ((rq->current_nr_sectors==1) ^ ((stat & DRQ_STAT)!=0)) { (tomita) d) same as c if ((rq->current_nr_sectors==1) != ((stat & DRQ_STAT)!=0)) { (linus)
Note: DRQ_STAT will be set when this routine is entered per "PIO Out protocol spec".
c+d will deadlock. (it will _not_ send the last sector)
a+b will send the last sector to the drive when it isn't ready to receive. (Although most of the time it _will_ be ready when we enter this routine)
So: if( (stat & DRQ_STAT) && !(stat & BSY_STAT)) { should be correct.
The original intent for a) was probably: check DRQ only on the _first_ sector we transfer as shown in "ATA-4 PIO data out command protocol", which would translate to: e) if ( !(rq->are_we_transferring_the_first_sector_just_now) || ((stat & DRQ_STAT)!=0)) {
- Gunther
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