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machine: make sure DMA buffers do not escape unnecessarily

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Writing the pointer of a buffer to memory-mapped I/O will normally cause
it to escape, which forces the compiler to heap-allocate the buffer. But
we do know how long the value stays alive, so we can tell the compiler
to keep it alive exactly until it is not needed anymore - and tell it to
not treat the pointer-to-uintptr cast as escaping.
This commit is contained in:
Ayke van Laethem
2025-06-20 09:48:25 +02:00
committed by Ron Evans
parent 5ae8fd1f6f
commit b203314c2f
7 changed files with 118 additions and 17 deletions
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src/machine/machine_nrf528xx.go
+17 -4
View File
@@ -49,7 +49,7 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) (err error) {
// Configure for a single shot to perform both write and read (as applicable)
if len(w) != 0 {
i2c.Bus.TXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&w[0]))))
i2c.Bus.TXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(w)))))
i2c.Bus.TXD.MAXCNT.Set(uint32(len(w)))
// If no read, immediately signal stop after TX
@@ -58,7 +58,7 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) (err error) {
}
}
if len(r) != 0 {
i2c.Bus.RXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&r[0]))))
i2c.Bus.RXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(r)))))
i2c.Bus.RXD.MAXCNT.Set(uint32(len(r)))
// Auto-start Rx after Tx and Stop after Rx
@@ -89,6 +89,11 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) (err error) {
}
}
// Make sure the w and r buffers stay alive until this point, so they won't
// be garbage collected while the buffers are used by the hardware.
keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(w)))
keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(r)))
return
}
@@ -117,7 +122,7 @@ func (i2c *I2C) Listen(addr uint8) error {
//
// For request events, the caller MUST call `Reply` to avoid hanging the i2c bus indefinitely.
func (i2c *I2C) WaitForEvent(buf []byte) (evt I2CTargetEvent, count int, err error) {
i2c.BusT.RXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&buf[0]))))
i2c.BusT.RXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(buf)))))
i2c.BusT.RXD.MAXCNT.Set(uint32(len(buf)))
i2c.BusT.TASKS_PREPARERX.Set(nrf.TWIS_TASKS_PREPARERX_TASKS_PREPARERX_Trigger)
@@ -134,6 +139,10 @@ func (i2c *I2C) WaitForEvent(buf []byte) (evt I2CTargetEvent, count int, err err
}
}
// Make sure buf stays alive until this point, so it won't be garbage
// collected while it is used by the hardware.
keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(buf)))
count = 0
evt = I2CFinish
err = nil
@@ -163,7 +172,7 @@ func (i2c *I2C) WaitForEvent(buf []byte) (evt I2CTargetEvent, count int, err err
// Reply supplies the response data the controller.
func (i2c *I2C) Reply(buf []byte) error {
i2c.BusT.TXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&buf[0]))))
i2c.BusT.TXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(buf)))))
i2c.BusT.TXD.MAXCNT.Set(uint32(len(buf)))
i2c.BusT.EVENTS_STOPPED.Set(0)
@@ -180,6 +189,10 @@ func (i2c *I2C) Reply(buf []byte) error {
}
}
// Make sure the buffer stays alive until this point, so it won't be garbage
// collected while it is used by the hardware.
keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(buf)))
i2c.BusT.EVENTS_STOPPED.Set(0)
return nil