Subject: Re: com rumblings...
To: None <>
From: Christos Zoulas <>
List: tech-kern
Date: 06/16/2006 15:04:03
In article <>,
Garrett D'Amore  <> wrote:
>Charles M. Hannum wrote:
>> On Thu, Jun 15, 2006 at 01:17:06PM -0700, Garrett D'Amore wrote:
>>> Wouldn't those 4- or 6- (or more!) port cards generally have working
>>> FIFOs on them?
>> Sure, but that only helps so much.  At 115200, you're still talking
>> about 1000 interrupts/sec (you can't just divide by 16) per port, per
>> direction.  And that's assuming no control signals are being toggled.
>> On a 386, this added up very quickly.  That's why the "hard" interrupt
>> routine had the absolute highest priority, even outside splhigh().  It
>> worked, at the time, before a lot of crap was added in the path.  Also,
>> even in the mid-90s, people were starting to "overclock" com-like parts
>> to 230400 and 460800 bps.
>Yes, I know about some of that.  I've been working on a Solaris driver
>for 16950s, that uses a 128-byte fifo to help out somewhat.  I've seen
>other chips that use DMA to minimize the time spent doing PIO, as well. 
>(Alchemy Au15xx cpus can use DMA to get to 4Mbps.)
>Anyway, I don't think anything I've done fundamentally changes the
>paths, other than possibly requiring an extra indirect memory
>reference.  In theory, a good optimizer could even remove that, though
>we could need to add some hints about the fact that the bus handles and
>tags are not changing so can be safely cached.  (Though, again, a good
>optimizer should be able to figure this out since the routines at issue
>are leaf routines.)
>I'm not sure how good gcc's optimizer is at detecting that:
>    struct {
>       bus_space_handle_t h;
>       bus_space_tag_t   t;
>    } s;
>    bus_space_write_1(s->t, s->h, OFFSET1, VAL1);
>    /* possibly insert code that doesn't modify s */
>    bus_space_write_1(s->t, s->h, OFFSET2, VAL2);
>both values of s->t and s->h are really the same and could be cached in
>registers without having to dereference thru s for each
>bus_space_write_1 call.
>whether bus_space_xxx are macros or functions probably also plays a role
>in this.

Would "const" help here?