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[Bug 1905226] Re: intel-hda: stream reset bits are broken

From: Launchpad Bug Tracker
Subject: [Bug 1905226] Re: intel-hda: stream reset bits are broken
Date: Sat, 10 Jul 2021 04:17:19 -0000

[Expired for QEMU because there has been no activity for 60 days.]

** Changed in: qemu
       Status: Incomplete => Expired

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  intel-hda: stream reset bits are broken

Status in QEMU:

Bug description:
  From HD audio spec, section 3.3.35:

  "Stream Reset (SRST): Writing a 1 causes the corresponding stream to
  be reset. [...] After the stream hardware has completed sequencing
  into the reset state, it will report a 1 in this bit. Software must
  read a 1 from this bit to verify that the stream is in reset. Writing
  a 0 causes the corresponding stream to exit reset. When the stream
  hardware is ready to begin operation, it will report a 0 in this bit.
  Software must read a 0 from this bit before accessing any of the
  stream registers."

  So to reset a stream I set the bit, but it never reads back as 1 so
  the driver either times out or will hang forever waiting for it to
  become 1. I looked into why this happens and found that as of the
  latest version (8110fa1), in function intel_hda_set_st_ctl() of the

      if (st->ctl & 0x01) {
          /* reset */
          dprint(d, 1, "st #%d: reset\n", reg->stream);
          st->ctl = SD_STS_FIFO_READY << 24;

  This causes the bit to immediately become set to 0 even if I write a
  1, and clearly does not meet the spec. I checked behaviour of real
  hardware and it works as expected, i.e. I see the bit will become 1
  and 0 when I write to it.

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