ata: Advertise multiword DMA support - #214
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Followup to dingusdev#192. That implemented DMA support, but did not advertise it via the IDENTIFY command. While the 10.0 public beta used it unconditionally, later releases will only use it if the hardware reports the MWDMA bit. Takes booting 10.3 (to WindowServer startup) from ~60 seconds to ~47 seconds on my machine.
mihaip
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With dingusdev#214 we started to advertise the IDE DMA support from dingusdev#192 to guests. That appeared to break booting Mac OS 8.x, at least on the Beige G3: the startup disk was not even detected. Mac OS 8.1 sets up an IDE DMA read as INPUT_MORE, then a NOP with its i field set to always, then STOP, which clears the channel's ACTIVE bit. The emulated transfer completion path interpreted only one following descriptor, so it stopped at NOP. NOP raised the interrupt, but STOP was never reached and the channel stayed active. The Mac OS X driver is more tolerant of this, which is why it was not an issue there. The DBDMA Specification, section 1.4, says: "The target fetches command entries and performs the command-entry-specified data-transfer operation, processing command entries until a STOP entry is reached". Keep interpreting ready commands until a transfer blocks or the channel stops, as start() and resume() already did. This is closer to the hardware behavior, and empirically lets Mac OS 8.1 boot again.
dingusdev
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Aug 22, 2026
With #214 we started to advertise the IDE DMA support from #192 to guests. That appeared to break booting Mac OS 8.x, at least on the Beige G3: the startup disk was not even detected. Mac OS 8.1 sets up an IDE DMA read as INPUT_MORE, then a NOP with its i field set to always, then STOP, which clears the channel's ACTIVE bit. The emulated transfer completion path interpreted only one following descriptor, so it stopped at NOP. NOP raised the interrupt, but STOP was never reached and the channel stayed active. The Mac OS X driver is more tolerant of this, which is why it was not an issue there. The DBDMA Specification, section 1.4, says: "The target fetches command entries and performs the command-entry-specified data-transfer operation, processing command entries until a STOP entry is reached". Keep interpreting ready commands until a transfer blocks or the channel stops, as start() and resume() already did. This is closer to the hardware behavior, and empirically lets Mac OS 8.1 boot again.
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Followup to #192. That implemented DMA support, but did not advertise it via the IDENTIFY command. While the 10.0 public beta used it unconditionally, later releases will only use it if the hardware reports the MWDMA bit.
DMA is more efficient than PIO (less time spent in the CPU emulator); this takes booting 10.3 (to WindowServer startup) from ~60 seconds to ~47 seconds on my machine.