[PATCH 4/9] hw/ide/ahci: simplify and document PxCI handling

[Niklas Cassel <nks@flawful.org>]

From: Niklas Cassel <niklas.cassel@wdc.com>

The AHCI spec states that:
For NCQ, PxCI is cleared on command queued successfully.

For non-NCQ, PxCI is cleared on command completed successfully.
(A non-NCQ command that completes with error does not clear PxCI.)

The current QEMU implementation either clears PxCI in check_cmd(),
or in ahci_cmd_done().

check_cmd() will clear PxCI for a command if handle_cmd() returns 0.
handle_cmd() will return -1 if BUSY or DRQ is set.

The QEMU implementation for NCQ commands will currently not set BUSY
or DRQ, so they will always have PxCI cleared by handle_cmd().
ahci_cmd_done() will never even get called for NCQ commands.

Non-NCQ commands are executed by ide_bus_exec_cmd().
Non-NCQ commands in QEMU are implemented either in a sync or in an async
way.

For non-NCQ commands implemented in a sync way, the command handler will
return true, and when ide_bus_exec_cmd() sees that a command handler
returns true, it will call ide_cmd_done() (which will call
ahci_cmd_done()). For a command implemented in a sync way,
ahci_cmd_done() will do nothing (since busy_slot is not set). Instead,
after ide_bus_exec_cmd() has finished, check_cmd() will clear PxCI for
these commands.

For non-NCQ commands implemented in an async way (using either aiocb or
pio_aiocb), the command handler will return false, ide_bus_exec_cmd()
will not call ide_cmd_done(), instead it is expected that the async
callback function will call ide_cmd_done() once the async command is
done. handle_cmd() will set busy_slot, if and only if BUSY or DRQ is
set, and this is checked _after_ ide_bus_exec_cmd() has returned.
handle_cmd() will return -1, so check_cmd() will not clear PxCI.
When the async callback calls ide_cmd_done() (which will call
ahci_cmd_done()), it will see that busy_slot is set, and
ahci_cmd_done() will clear PxCI.

This seems racy, since busy_slot is set _after_ ide_bus_exec_cmd() has
returned. The callback might come before busy_slot gets set. And it is
quite confusing that ahci_cmd_done() will be called for all non-NCQ
commands when the command is done, but will only clear PxCI in certain
cases, even though it will always write a D2H FIS and raise an IRQ.

Even worse, in the case where ahci_cmd_done() does not clear PxCI, it
still raises an IRQ. Host software might thus read an old PxCI value,
since PxCI is cleared (by check_cmd()) after the IRQ has been raised.

Try to simplify this by always setting busy_slot for non-NCQ commands,
such that ahci_cmd_done() will always be responsible for clearing PxCI
for non-NCQ commands.

For NCQ commands, clear PxCI when we receive the D2H FIS, but before
raising the IRQ, see AHCI 1.3.1, section 5.3.8, states RegFIS:Entry and
RegFIS:ClearCI.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
---
 hw/ide/ahci.c | 70 ++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 50 insertions(+), 20 deletions(-)

diff --git a/hw/ide/ahci.c b/hw/ide/ahci.c
index 62aebc8de7..9d79b071b8 100644
--- a/hw/ide/ahci.c
+++ b/hw/ide/ahci.c
@@ -41,9 +41,10 @@
 #include "trace.h"
 
 static void check_cmd(AHCIState *s, int port);
-static int handle_cmd(AHCIState *s, int port, uint8_t slot);
+static void handle_cmd(AHCIState *s, int port, uint8_t slot);
 static void ahci_reset_port(AHCIState *s, int port);
 static bool ahci_write_fis_d2h(AHCIDevice *ad, bool d2h_fis_i);
+static void ahci_clear_cmd_issue(AHCIDevice *ad, uint8_t slot);
 static void ahci_init_d2h(AHCIDevice *ad);
 static int ahci_dma_prepare_buf(const IDEDMA *dma, int32_t limit);
 static bool ahci_map_clb_address(AHCIDevice *ad);
@@ -591,9 +592,8 @@ static void check_cmd(AHCIState *s, int port)
 
     if ((pr->cmd & PORT_CMD_START) && pr->cmd_issue) {
         for (slot = 0; (slot < 32) && pr->cmd_issue; slot++) {
-            if ((pr->cmd_issue & (1U << slot)) &&
-                !handle_cmd(s, port, slot)) {
-                pr->cmd_issue &= ~(1U << slot);
+            if (pr->cmd_issue & (1U << slot)) {
+                handle_cmd(s, port, slot);
             }
         }
     }
@@ -1123,6 +1123,22 @@ static void process_ncq_command(AHCIState *s, int port, const uint8_t *cmd_fis,
         return;
     }
 
+    /*
+     * A NCQ command clears the bit in PxCI after the command has been QUEUED
+     * successfully (ERROR not set, BUSY and DRQ cleared).
+     *
+     * For NCQ commands, PxCI will always be cleared here.
+     *
+     * (Once the NCQ command is COMPLETED, the device will send a SDB FIS with
+     * the interrupt bit set, which will clear PxSACT and raise an interrupt.)
+     */
+    ahci_clear_cmd_issue(ad, slot);
+
+    /*
+     * In reality, for NCQ commands, PxCI is cleared after receiving a D2H FIS
+     * without the interrupt bit set, but since ahci_write_fis_d2h() can raise
+     * an IRQ on error, we need to call them in reverse order.
+     */
     ahci_write_fis_d2h(ad, false);
 
     ncq_tfs->used = 1;
@@ -1197,6 +1213,7 @@ static void handle_reg_h2d_fis(AHCIState *s, int port,
 {
     IDEState *ide_state = &s->dev[port].port.ifs[0];
     AHCICmdHdr *cmd = get_cmd_header(s, port, slot);
+    AHCIDevice *ad = &s->dev[port];
     uint16_t opts = le16_to_cpu(cmd->opts);
 
     if (cmd_fis[1] & 0x0F) {
@@ -1273,11 +1290,19 @@ static void handle_reg_h2d_fis(AHCIState *s, int port,
     /* Reset transferred byte counter */
     cmd->status = 0;
 
+    /*
+     * A non-NCQ command clears the bit in PxCI after the command has COMPLETED
+     * successfully (ERROR not set, BUSY and DRQ cleared).
+     *
+     * For non-NCQ commands, PxCI will always be cleared by ahci_cmd_done().
+     */
+    ad->busy_slot = slot;
+
     /* We're ready to process the command in FIS byte 2. */
     ide_bus_exec_cmd(&s->dev[port].port, cmd_fis[2]);
 }
 
-static int handle_cmd(AHCIState *s, int port, uint8_t slot)
+static void handle_cmd(AHCIState *s, int port, uint8_t slot)
 {
     IDEState *ide_state;
     uint64_t tbl_addr;
@@ -1288,12 +1313,12 @@ static int handle_cmd(AHCIState *s, int port, uint8_t slot)
     if (s->dev[port].port.ifs[0].status & (BUSY_STAT|DRQ_STAT)) {
         /* Engine currently busy, try again later */
         trace_handle_cmd_busy(s, port);
-        return -1;
+        return;
     }
 
     if (!s->dev[port].lst) {
         trace_handle_cmd_nolist(s, port);
-        return -1;
+        return;
     }
     cmd = get_cmd_header(s, port, slot);
     /* remember current slot handle for later */
@@ -1303,7 +1328,7 @@ static int handle_cmd(AHCIState *s, int port, uint8_t slot)
     ide_state = &s->dev[port].port.ifs[0];
     if (!ide_state->blk) {
         trace_handle_cmd_badport(s, port);
-        return -1;
+        return;
     }
 
     tbl_addr = le64_to_cpu(cmd->tbl_addr);
@@ -1312,7 +1337,7 @@ static int handle_cmd(AHCIState *s, int port, uint8_t slot)
                              DMA_DIRECTION_TO_DEVICE, MEMTXATTRS_UNSPECIFIED);
     if (!cmd_fis) {
         trace_handle_cmd_badfis(s, port);
-        return -1;
+        return;
     } else if (cmd_len != 0x80) {
         ahci_trigger_irq(s, &s->dev[port], AHCI_PORT_IRQ_BIT_HBFS);
         trace_handle_cmd_badmap(s, port, cmd_len);
@@ -1336,15 +1361,6 @@ static int handle_cmd(AHCIState *s, int port, uint8_t slot)
 out:
     dma_memory_unmap(s->as, cmd_fis, cmd_len, DMA_DIRECTION_TO_DEVICE,
                      cmd_len);
-
-    if (s->dev[port].port.ifs[0].status & (BUSY_STAT|DRQ_STAT)) {
-        /* async command, complete later */
-        s->dev[port].busy_slot = slot;
-        return -1;
-    }
-
-    /* done handling the command */
-    return 0;
 }
 
 /* Transfer PIO data between RAM and device */
@@ -1498,6 +1514,16 @@ static int ahci_dma_rw_buf(const IDEDMA *dma, bool is_write)
     return 1;
 }
 
+static void ahci_clear_cmd_issue(AHCIDevice *ad, uint8_t slot)
+{
+    IDEState *ide_state = &ad->port.ifs[0];
+
+    if (!(ide_state->status & (BUSY_STAT | DRQ_STAT))) {
+        ad->port_regs.cmd_issue &= ~(1 << slot);
+    }
+}
+
+/* Non-NCQ command is done - This function is never called for NCQ commands. */
 static void ahci_cmd_done(const IDEDMA *dma)
 {
     AHCIDevice *ad = DO_UPCAST(AHCIDevice, dma, dma);
@@ -1506,11 +1532,15 @@ static void ahci_cmd_done(const IDEDMA *dma)
 
     /* no longer busy */
     if (ad->busy_slot != -1) {
-        ad->port_regs.cmd_issue &= ~(1 << ad->busy_slot);
+        ahci_clear_cmd_issue(ad, ad->busy_slot);
         ad->busy_slot = -1;
     }
 
-    /* update d2h status */
+    /*
+     * In reality, for non-NCQ commands, PxCI is cleared after receiving a D2H
+     * FIS with the interrupt bit set, but since ahci_write_fis_d2h() will raise
+     * an IRQ, we need to call them in reverse order.
+     */
     ahci_write_fis_d2h(ad, true);
 
     if (ad->port_regs.cmd_issue && !ad->check_bh) {
-- 
2.40.0