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Subject: Re: [PATCH v2] virtio-mmio: Specify wait needed in driver during reset
* Michael S. Tsirkin <mst@redhat.com> [2021-08-16 01:30:57]: > > +After writing 0 to \field{Status}, which triggers a reset of device, the driver MUST wait for a read of > > +\field{Status} to return 0 before proceeding with the remaining steps of initializing the device. > > I know you copied this from pci but looking at PCI now, I think > this is not great there either. > > 1. This is IMHO too opaque in that we did not say driver should read. > > E.g. after writing 0 to Status and before reading any fields, the driver > MUST read Status and wait for a read etc. > > also would not just a read be sufficient? Is there a case for device to return any value > other than 0 to signal "reset in progress"? Not sure I follow you here. We need some means for device to indicate its reset operation is complete - non-zero value of Status would indicate reset is still in progress while zero value would indicate reset is complete? > 2. what if device encounters an error and sets > NEED_RESET meanwhile? waiting for a reset will then deadlock ... > I know, it's problematic like this in PCI too. Hmm a reset itself having failed - not sure if another reset will clear things up better. I guess device could attempt another reset by itself, but may give up after couple of attempts by indicating NEED_RESET, at which point not sure if another driver initiated reset will make a difference. At the minimum, the guidance for drivers here would be to block until reset is completed (indicated by Status returning 0) or device encountering some failure (NEED_RESET)? When NEED_RESET is seen, should the spec prescribe driver to attempt reset again? > Further what will device return after reset was written but > before it completed? I think it is better to specify that > rather than rely on any non-0 value meaning "wait for reset". Yes agree, we should have a specific status bit to indicate "reset in progress" > For pci maybe make it a SHOULD ... Do you mean pci driver SHOULD wait until this new status bit (RESET_IN_PROGRESS) is cleared? - vatsa --- Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a non-member to the virtio-dev mailing list for consideration and inclusion.
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