On Thu, Apr 03, 2008 at 07:49:09PM -0400, David Zeuthen wrote:
On Thu, 2008-04-03 at 10:38 -0400, Doug Goldstein wrote:Now I could be wrong (I'm not too well versed in hardware specs and driver bits). I was under the impression that MMC-2 ATAPI provides for a Media Status Notification. Which is a polled condition. However, the device should emit an Unit Attention Condition when a disc *may* have been removed. Which basically means, the device should emit a notice when the drive door opens and closes to tell you that the media may have changed.As the bug report explains now, HAL is checking for the drive door to be closed. Instead of relying on the Unit Attention Condition notice.Except that approximately 0% of the drives out there support that command properly. See 'man hal-disable-polling' for the longer story.
I don't know why it causes fewer wakeups, but it does. It may or may not have anything to do with MMC-2 ATAPI. HAL shows ten wakeups per second without the patch, and doesn't appear in powertop with the patch. It doesn't really matter to me why it happens, but since I'm now curious: the cdrom driver talks to the specific driver (in the example I'm using, ide-cd) and does a media_changed request, which calls ide_cdrom_check_media_change_real. That does a cdrom_check_status, which issues a TEST_UNIT_READY command.
Issuing a drive status command calls ide_cdrom_drive_status, which also eventually issues a cdrom_check_status, but it also calls cdrom_get_media_event, which does a GET_EVENT_STATUS_NOTIFICATION. This appears to be the expensive part. ide_cdrom_drive_status also checks the MMC-2 ATAPI method with UNIT_ATTENTION, and doesn't bother doing the more expensive check if that happens.
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