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q35: five seconds of boot spent waiting on disks that are not there

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/proc/interrupts on q35 said "LOSING 526" ticks: 5 s of boot with
interrupts off. Timing each step of kmain() found two waits:

- AHCI sent a COMRESET to every empty port and waited a second for a
  link. DET = 0 is "no device", so only DET = 1 is worth a reset now
  (after 20 ms of grace for a drive that is still spinning up).
- ideinit() read 0xFF from a bus with no IDE controller, took the BSY
  bit for a busy drive, and waited out its 1 s timeout. A status of
  0xFF means no controller and is answered at once.

That is every PC with SATA disks. q35 now loses 33 ticks, under the
flag; i440fx still roots on IDE and loses none.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HH7xukt4aaiGj1nQEAoQf1
This commit is contained in:
Nexoniarz 2026-10-22 11:10:07 +02:00
commit c1e52e172f
2 changed files with 25 additions and 0 deletions
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12
kernel/dev/ide.c
View file

@ -130,6 +130,18 @@ void
ideinit(void)
{
initlock(&idelock, "ide");
// No controller at all: the bus floats and the status port reads 0xFF.
// That has BSY set, so idewait() below took it for a drive that was still
// busy and waited out its full second -- a second of every boot on q35 and
// on any PC whose disks are SATA, which is all of them. 0xFF is not a
// status any drive reports (BSY and DRDY are never both set), so it can be
// answered at once.
if(inb(0x1f7) == 0xFF){
kinfo("ide: no controller at the legacy ports\n");
return;
}
// ISA signalling: edge-triggered, active high. Everything at a fixed
// legacy port is wired this way; PCI devices are not.
irq_register(IRQ_IDE, ide_irq, 0, "ide", 0);

13
kernel/drivers/storage/ahci/ahci.c
View file

@ -167,9 +167,22 @@ port_stop(volatile uint8 *r)
static int
port_link(volatile uint8 *r)
{
// A drive told to spin up a moment ago takes a few milliseconds to be seen
// at all; give it twenty before calling the port empty.
uint32 ssts = rd(r, PX_SSTS);
for (int i = 0; i < 20 && (ssts & 0xF) == 0; i++) {
microdelay(1000);
ssts = rd(r, PX_SSTS);
}
if ((ssts & 0xF) == 3)
return 1;
// DET = 0 is "no device detected": an empty port, not a slow one. This
// used to COMRESET every empty port and wait a second for an answer that
// could not come -- four seconds of boot on a q35 with one disk, all of it
// with interrupts off. Only a device that is detected without a link
// (DET = 1) is worth the reset.
if ((ssts & 0xF) != 1)
return 0;
wr(r, PX_SCTL, (rd(r, PX_SCTL) & ~0xFu) | 1); // DET = 1: COMRESET
microdelay(2000); // at least 1 ms