On 10/19/2017 09:55 AM, Marc Kleine-Budde wrote: > On 10/19/2017 03:54 PM, Franklin S Cooper Jr wrote: >> On 10/19/2017 06:32 AM, Marc Kleine-Budde wrote: >>> On 10/19/2017 01:09 PM, Sekhar Nori wrote: >>>> On Thursday 19 October 2017 02:43 PM, Marc Kleine-Budde wrote: >>>>> On 10/19/2017 07:07 AM, Sekhar Nori wrote: >>>>>>>>> Sounds reasonable. What's the status of this series? >>>>>>>> >>>>>>>> I have had some offline discussions with Franklin on this, and I am not >>>>>>>> fully convinced that DT is the way to go here (although I don't have >>>>>>>> the >>>>>>>> agreement with Franklin there). >>>>>>> >>>>>>> Probably the fundamental area where we disagree is what "default" SSP >>>>>>> value should be used. Based on a short (< 1 ft) point to point test >>>>>>> using a SSP of 50% worked fine. However, I'm not convinced that this >>>>>>> default value of 50% will work in a more "traditional" CAN bus at higher >>>>>>> speeds. Nor am I convinced that a SSP of 50% will work on every MCAN >>>>>>> board in even the simplest test cases. >>>>>>> >>>>>>> I believe that this default SSP should be a DT property that allows any >>>>>>> board to determine what default value works best in general. >>>>>> >>>>>> With that, I think, we are taking DT from describing board/hardware >>>>>> characteristics to providing default values that software should use. >>>>> >>>>> If the default value is board specific and cannot be calculated in >>>>> general or from other values present in the DT, then it's from my point >>>>> of view describing the hardware. >>>>> >>>>>> In any case, if Marc and/or Wolfgang are okay with it, binding >>>>>> documentation for such a property should be sent to DT maintainers for >>>>>> review. >>>>>> >>>>>>>> >>>>>>>> There are two components in configuring the secondary sample point. It >>>>>>>> is the transceiver loopback delay and an offset (example half of the >>>>>>>> bit >>>>>>>> time in data phase). >>>>>>>> >>>>>>>> While the transceiver loopback delay is pretty board dependent (and >>>>>>>> thus >>>>>>>> amenable to DT encoding), I am not quite sure the offset can be >>>>>>>> configured in DT because its not really board dependent. >>>>>>>> >>>>>>>> Unfortunately, offset calculation does not seem to be an exact science. >>>>>>>> There are recommendations ranging from using 50% of bit time to making >>>>>>>> it same as the sample point configured. This means users who need to >>>>>>>> change the SSP due to offset variations need to change their DT even >>>>>>>> without anything changing on their board. >>>>>>>> >>>>>>>> Since we have a netlink socket interface to configure sample point, I >>>>>>>> wonder if that should be extended to configure SSP too (or at least the >>>>>>>> offset part of SSP)? >>>>>>> >>>>>>> Sekhar is right that ideally the user should be able to set the SSP at >>>>>>> runtime. However, my issue is that based on my experience CAN users >>>>>>> expect the driver to just work the majority of times. For unique use >>>>>>> cases where the driver calculated values don't work then the user should >>>>>>> be able to override it. This should only be done for a very small >>>>>>> percentage of CAN users. Unless you allow DT to provide a default SSP >>>>>>> many users of MCAN may find that the default SSP doesn't work and must >>>>>>> always use runtime overrides to get anything to work. I don't think that >>>>>>> is a good user experience which is why I don't like the idea. >>>>>> >>>>>> Fair enough. But not quite sure if CAN users expect CAN-FD to "just >>>>>> work" without doing any bittiming related setup. >>>>> >>>>> From my point of view I'd rather buy a board from a HW vendor where >>>>> CAN-FD works, rather than a board where I have to tweak the bit-timing >>>>> for a simple CAN-FD test setup. >>>>> >>>>> As far as I see for the m_can driver it's a single tuple: "bitrate > 2.5 >>>>> MBit/s -> 50%". Do we need an array of tuples in general? >> >> Internally what I proposed was a binding that allowed you to pass in an >> array of a range of baud rates and then a SSP for that baud rate range. >> Therefore, if the baud rate being used impacted what SSP worked then it >> allows someone to provide a range of defaults. Of course a person also >> has the ability to use a single large range thus implementing a single >> default SSP value. > > A single tuple is just a special case of a list of tuples :) > > I was thinking of something like this: > > First we need a struct defining the bitrate spp relationship. > > The driver provides default values by a sorted array of these structs > together with array length. > > During device registration we assign these default values to the actual > driver instance. > > The netlink code can read and overwrite the current values. Maybe it can > read the default values. > > The bitrate calculation code calculates the to be used spp value. > > The driver sets the value during the open callback. > >>> Do we need more than one tuple here? >>> >>>>> If good default values are transceiver and board specific, they can go >>>>> into the DT. We need a generic (this means driver agnostic) binding for >>>>> this. If this table needs to be tweaked for special purpose, then we can >>>>> add a netlink interface for this as well. >>>>> >>>>> Comments? >>>> >>>> I dont know how a good default (other than 50% as the starting point) >>>> can be arrived at without doing any actual measurements on the actual >>>> network. Since we do know that the value has to be tweaked, agree that >>>> netlink interface has to be provided. >> >> Now I have seen in non public documentations that setting SP to SSP also >> works. > > This can already by done now, without the need for new interfaces. > >> This makes a bit more sense to me and I'm alot more comfortable going >> with this. However, since its based on non public information I can't >> justify it beyond that "it works for me". But I'm alot more >> comfortable going with then saying "hey this default value works for >> TI's dra76 evm. Therefore, every MCAN board will be stuck by default >> for a value that works for us". So if there is no push back with >> going with SSP = db SP with no documentation to back up why that is >> being used then I will try that out and send patches. > > This means we postpone the whole add-new-interface-dance until the > SPP=SP approach doesn't work for some usecase?
My justification behind using SSP = SP will be "it works during my test". If that is fine then I am ok with postponing any new bindings. > > Marc >
