Messages in this thread | | | Date | Wed, 29 Apr 2015 14:08:14 +0100 | From | Sudeep Holla <> | Subject | Re: [PATCH 1/4] mailbox: add support for System Control and Power Interface(SCPI) protocol |
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On 29/04/15 13:25, Jon Medhurst (Tixy) wrote: > On Wed, 2015-04-29 at 12:43 +0100, Jon Medhurst (Tixy) wrote: >> On Wed, 2015-04-29 at 11:53 +0100, Sudeep Holla wrote: >>> On 28/04/15 14:54, Jon Medhurst (Tixy) wrote: >>>> On Mon, 2015-04-27 at 12:40 +0100, Sudeep Holla wrote: >> [...] >>>>> + int ret; >>>>> + u8 token, chan; >>>>> + struct scpi_xfer *msg; >>>>> + struct scpi_chan *scpi_chan; >>>>> + >>>>> + chan = atomic_inc_return(&scpi_info->next_chan) % scpi_info->num_chans; >>>>> + scpi_chan = scpi_info->channels + chan; >>>>> + >>>>> + msg = get_scpi_xfer(scpi_chan); >>>>> + if (!msg) >>>>> + return -ENOMEM; >>>>> + >>>>> + token = atomic_inc_return(&scpi_chan->token) & CMD_TOKEN_ID_MASK; >>>> >>>> So, this 8 bit token is what's used to 'uniquely' identify a pending >>>> command. But as it's just an incrementing value, then if one command >>>> gets delayed for long enough that 256 more are issued then we will have >>>> a non-unique value and scpi_process_cmd can go wrong. >>>> >>> >>> IMO by the time 256 message are queued up and serviced we would timeout >>> on the initial command. Moreover the core mailbox has sent the mailbox >>> length to 20(MBOX_TX_QUEUE_LEN) which needs to removed to even get the >>> remote chance of hit the corner case. >> >> The corner case can be hit even if the queue length is only 2, because >> other processes/cpus can use the other message we don't own here and >> they can send then receive a message using that, 256 times. The corner >> case doesn't require 256 simultaneous outstanding requests. >> >> That is the reason I suggested that rather than using an incrementing >> value for the 'unique' token, that each message instead contain the >> value of the token to use with it. > > Of course, I failed to mention that this solution to this problem makes > thing worse for the situation where we timeout messages, because the > same token will get reused quicker in that case. So, in practice, if we > have timeouts, and a unchangeable protocol limitation of 256 tokens, > then using those tokens in order, for each message sent is probably the > best we can do. >
I agree, I think we must be happy with that for now :)
> Perhaps that's the clue, generate and add the token to the message, just > before transmission via the MHU, at a point where we know no other > request can overtake us. In scpi_tx_prepare? Perhaps, it might also be > good to only use up a token if we are expecting a response, and use zero > for other messages? > > Something like this totally untested patch... >
Looks good and best we can do with the limitations we have IMO
Regards, Sudeep
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