On Tuesday 14 June 2016 06:08 PM, Ivan Khoronzhuk wrote: > > > On 13.06.16 18:19, Andrew F. Davis wrote: >> On 06/13/2016 03:22 AM, Mugunthan V N wrote: >>> On Saturday 11 June 2016 04:34 AM, Schuyler Patton wrote: >>>> >>>> >>>> On 06/08/2016 07:03 PM, Ivan Khoronzhuk wrote: >>>>> >>>>> >>>>> On 09.06.16 02:11, Schuyler Patton wrote: >>>>>> >>>>>> >>>>>> On 06/08/2016 09:06 AM, Ivan Khoronzhuk wrote: >>>>>>> >>>>>>> >>>>>>> On 08.06.16 17:01, Ivan Khoronzhuk wrote: >>>>>>>> Hi Schuyer, >>>>>>>> >>>>>>>> On 07.06.16 18:26, Schuyler Patton wrote: >>>>>>>>> Hi, >>>>>>>>> >>>>>>>>> On 06/07/2016 08:59 AM, Ivan Khoronzhuk wrote: >>>>>>>>>> There is no reason in rx_descs property because davinici_cpdma >>>>>>>>>> driver splits pool of descriptors equally between tx and rx >>>>>>>>>> channels. >>>>>>>>>> So, this patch series makes driver to use available number of >>>>>>>>>> descriptors for rx channels. >>>>>>>>> >>>>>>>>> I agree with the idea of consolidating how the descriptors are >>>>>>>>> defined because of >>>>>>>>> the two variable components, number and size of the pool can be >>>>>>>>> confusing to >>>>>>>>> end users. I would like to request to change how it is being >>>>>>>>> proposed here. >>>>>>>>> >>>>>>>>> I think the number of descriptors should be left in the device >>>>>>>>> tree source file as >>>>>>>>> is and remove the BD size variable and have the driver calculate >>>>>>>>> the size of the >>>>>>>>> pool necessary to support the descriptor request. From an user >>>>>>>>> perspective it is >>>>>>>>> easier I think to be able to list the number of descriptors >>>>>>>>> necessary vs. the size >>>>>>>>> of the pool. >>>>>>>>> >>>>>>>>> Since the patch series points out how it is used so in the driver >>>>>>>>> so to make that >>>>>>>>> consistent is perhaps change rx_descs to total_descs. >>>>>>>>> >>>>>>>>> Regards, >>>>>>>>> Schuyler >>>>>>>> >>>>>>>> The DT entry for cpsw doesn't have property for size of the pool. >>>>>>>> It contains only BD ram size, if you mean this. The size of the >>>>>>>> pool is >>>>>>>> software decision. Current version of DT entry contain only rx desc >>>>>>>> number. >>>>>>>> That is not correct, as it depends on the size of the descriptor, >>>>>>>> which is also >>>>>>>> h/w parameter. The DT entry has to describe only h/w part and >>>>>>>> shouldn't contain >>>>>>>> driver implementation details, and I'm looking on it from this >>>>>>>> perspective. >>>>>>>> >>>>>>>> Besides, rx_descs describes only rx number of descriptors, that are >>>>>>>> taken from >>>>>>>> the same pool as tx descriptors, and setting rx desc to some new >>>>>>>> value doesn't >>>>>>>> mean that rest of them are freed for tx. Also, I'm going to send >>>>>>>> series that >>>>>>>> adds multi channel support to the driver, and in this case, >>>>>>>> splitting of the >>>>>>>> pool will be more sophisticated than now, after what setting those >>>>>>>> parameters >>>>>>>> for user (he should do this via device tree) can be even more >>>>>>>> confusing. But, >>>>>>> should -> shouldn't >>>>>>> >>>>>>>> as it's supposed, it's software decision that shouldn't leak to the >>>>>>>> DT. >>>>>> >>>>>> If this rx-desc field is removed how will the number of descriptors >>>>>> be set? >>>>>> >>>>>> This field has been used to increase the number of descriptors so >>>>>> high >>>>>> volume short packets are not dropped due to descriptor exhaustion. >>>>>> The current >>>>>> default number of 64 rx descriptors is too low for gigabit networks. >>>>>> Some users >>>>>> have a strong requirement for zero loss of UDP packets setting this >>>>>> field to a >>>>>> larger number and setting the descriptors off-chip was a means to >>>>>> solve >>>>>> the problem. >>>>> The current implementation of cpdma driver splits descs num on 2 parts >>>>> equally. >>>>> Total number = 256, then 128 reserved for rx and 128 for tx, but >>>>> setting this to >>>>> 64, simply limits usage of reserved rx descriptors to 64, so that: >>>>> 64 rx descs, 128 tx descs and 64 are always present in the pool but >>>>> cannot be used, >>>>> (as new rx descriptor is allocated only after previous was freed). >>>>> That means, 64 rx descs are unused. In case of rx descriptor >>>>> exhaustion, an user can >>>>> set rx_descs to 128, for instance, in this case all descriptors will >>>>> be in use, but then question, >>>>> why intentionally limit number of rx descs, anyway rest 64 descs >>>>> cannot be used for other >>>>> purposes. In case of this patch, all rx descs are in use, and no need >>>>> to correct number >>>>> of rx descs anymore, use all of them....and it doesn't have impact on >>>>> performance, as >>>>> anyway, bunch of rx descs were simply limited by DT and unused. So, >>>>> probably, there is no >>>>> reason to worry about that. >>>> >>>> When we see this issue we set the descriptors to DDR and put a large >>>> number >>>> in the desc count. unfortunately I wish I could provide a number, >>>> usually the issue >>>> is a volume burst of short UDP packets. >>>> >>>>> >>>>> PS: >>>>> It doesn't concern this patch, but, which PPS makes rx descs to be >>>>> exhausted?... >>>>> (In this case "desc_alloc_fail" counter contains some value for rx >>>>> channel, >>>>> and can be read with "ethtool -S eth0". Also, the user will be WARNed >>>>> ON by the driver) >>>>> >>>>> it's interesting to test it, I'm worrying about, because in case of >>>>> multichannel, >>>>> the pool is split between all channels... they are throughput limited, >>>>> but >>>>> anyway, it's good to correlate the number of descs with throughput >>>>> assigned to >>>>> a channel, if possible. That has to be possible, if setting to 128 >>>>> helps, then >>>>> has to be value between 64 and 128 to make handling of rx packets fast >>>>> enough. >>>>> After what, can be calculated correlation between number of rx descs >>>>> and throughput >>>>> split between channels.... >>>> >>>> With gigabit networks 64 or 128 rx descriptors is not going to >>>> enough to >>>> fix the >>>> DMA overrun problem. Usually we set this number to an arbitrarily large >>>> 2000 >>>> descriptors in external DDR to demonstrate it is possible to not drop >>>> packets. All >>>> this does is move the problem higher up so that the drops occur in >>>> network >>>> stack if the ARM is overloaded. With the high speed networks I would >>>> like >>>> to propose that the descriptor pool or pools are moved to DDR by >>>> default. It would >>>> be nice to have some reconfigurability or set a pool size that reduces >>>> or eliminates >>>> the DMA issue that is seen in these types of applications. >>>> >>>> This test gets used a lot, which is to send very short UDP packets. >>>> If I >>>> have the math >>>> right, a 52 byte (64 byte with the inter-frame gap) UDP packet the >>>> default 64 >>>> descriptors gets consumed in roughly 33uS. There are the switch fifos >>>> which will also >>>> allow some headroom, but a user was dropping packets at the switch when >>>> they >>>> were bursting 360 packets at the processor on a gigabit link >>>> >>> >>> I too agree that rx-descs can be derived from the pool size and >>> descriptor size in driver itself. The current driver uses bd_ram_size to >>> set the pool size when the descriptors are placed in DDR which is wrong. >>> >>> Here I propose an idea to solve Schuyler's concern to keep the >>> descriptors in DDR when a system need more rx descriptors for lossless >>> UDB performance. >>> >>> The DT property rx-descs can be removed and add a new DT property >>> *pool_size* to add support for descriptors memory size in DDR and define >>> a pool size which the system needs for a network to have lossless UDP >>> transfers. >>> >> >> I second the pool_size property, but being purely a driver configuration >> setting based on expected network environment and not a HW description >> it should probably be a module parameter, not DT property. >> >> Andrew > > Agree, DT is not the best place for it. > And it be done with separate patch-set, as this patch solves rx_descs > issue, > not pool_size. > +Dave There were many instance where David Miller discouraged using module parameters (some links below), thats why I suggested DT property. I am okay with module parameter as well, Leaving the decision to Dave which way it can be implemented. - https://patchwork.ozlabs.org/patch/82882/ - http://lists.openwall.net/netdev/2014/02/17/6 Regards Mugunthan V N -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
