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AM335x musb driver problems when heavily loaded

Andrew Goodbody
Prodigy 140 points
Other Parts Discussed in Thread: AM3352

On my system, AM3352 with kernel 3.14.26 I am seeing the musb driver sometimes return -ENOSPC. This appears to be because it cannot find an endpoint for the current URB. From what I can see in the source it would be possible to queue the URB but this is not done for anything other than control or bulk endpoints. I guess that this was a simplification done when the musb driver was written.

I also see babble errors on rare occasions which I think is a result of the lack of bandwidth management.


Is there anyone working on either queuing of URBs when all the endpoints are in use and/or bandwidth tracking and management for the musb driver?

  • 0
    TI__Guru*** 147816 points

    Andrew,
    I never received an issue report similar to this. Can you please provide more detail about your USB use case, especially how those endpoints have been used? Maybe that will give a clue how to investigate the issue.

  • 0 in reply to Bin Liu
    Prodigy 140 points

    Hi Bin,


    We are doing USB/IP with multiple iPads and Apple Configurator. The failures occur when doing a 'Prepare' of the iPads. The failures occur most often when the iPads reset themselves and attempt to re-establish connection with the host. I have not established what is using all the endpoints in the controller but I believe that it will be mostly bulk transfers and that is not leaving any endpoints available for the interrupt transfers. There should not be any isochronous transfers involved at all.

    The problem is when musb_schedule in musb_host.c returns -ENOSPC

  • 0 in reply to Andrew Goodbody
    TI__Guru*** 147816 points

    Andrew,

    Is the topology multiple Ipad/configurator connected to MUSB via a USB hub? If so, please connect one of the devices directly to MUSB (or Linux PC), then run command 'lsusb -v -d <vid:pid>' on the host, and send me the log, I want to check the ipad/configurator descriptors. I suspect the USB devices do not use BULK endpoints.

    Please review musb_schedule(), it only returns -ENOSPC for non-BULK endpoint request while no endpoint available.

  • 0 in reply to Bin Liu
    Prodigy 140 points
    Bus 001 Device 007: ID 05ac:12ab Apple, Inc.
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    (Bus Powered)
  • 0 in reply to Andrew Goodbody
    TI__Guru*** 147816 points

    Andrew,

    As expected, this device uses multiple Interrupt and Isochronous endpoints, for those type of transfers, MUSB does not support dynamic endpoint allocation.

    MUSB has 15 IN and 15 OUT endpoints, As more such devices enumerated, the 15 IN/OUT endpoints will be used up, then -ENOSPC is returned for further endpoint allocation request.

    This is a system limitation, it can only simultaneously support limited devices which use Interrupt or Isochronous endpoints, I am not aware of any workaround. Other USB controllers, for example xHCI, also have the same limitation, xHCI has more endpoints available though.

  • 0 in reply to Bin Liu
    Prodigy 140 points
    But is it not actually a driver limitation? EP1(in,out) is used with queues but EP2-15 have no queue. Would it not be possible to queue URBs on the endpoints that are not reserved for bulk or control traffic ie EP2-15?
  • 0 in reply to Andrew Goodbody
    TI__Guru*** 147816 points
    Adding queue to the Interrupt or Isochronous endpoints does not solve the issue. Unlike the endpoints for Bulk or Control transfers, once an endpoint is allocated for a periodic transfer (Int or Isoch), it will never be freed until application terminates the transfer - typically by unplugging the device, so even if there was a such queue, the URBs in the queue would never be executed, because the endpoint is always running for the very first URB.
  • 0 in reply to Bin Liu
    Prodigy 140 points
    Thank you for this explanation.
    It is a shame that using endpoints in the hardware to offload from the CPU ends up severely restricting the number of USB devices that can be supported at a time. The limited endpoints in xHCI are also a problem for us although at least on Apple hardware there is a workaround for that.