Date:	Fri, 10 Jun 2016 18:04:32 -0500
From:	Schuyler Patton <spatton@...com>
To:	Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
	<mugunthanvnm@...com>, <linux-kernel@...r.kernel.org>
CC:	<grygorii.strashko@...com>, <linux-omap@...r.kernel.org>,
	<netdev@...r.kernel.org>, <robh+dt@...nel.org>,
	<pawel.moll@....com>, <mark.rutland@....com>,
	<ijc+devicetree@...lion.org.uk>, <galak@...eaurora.org>,
	<bcousson@...libre.com>, <tony@...mide.com>,
	<devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] net: ethernet: ti: cpsw: delete rx_descs property



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

>
>>
>>>>
>>>>
>>>>>>
>>>>>> Based on master branch
>>>>>>
>>>>>> Since v1:
>>>>>> - separate device tree and driver patches
>>>>>> - return number of rx buffers from cpdma driver
>>>>>>
>>>>>> Ivan Khoronzhuk (2):
>>>>>>    net: ethernet: ti: cpsw: remove rx_descs property
>>>>>>    Documentation: DT: cpsw: remove rx_descs property
>>>>>>
>>>>>>   Documentation/devicetree/bindings/net/cpsw.txt |  1 -
>>>>>>   arch/arm/boot/dts/am33xx.dtsi                  |  1 -
>>>>>>   arch/arm/boot/dts/am4372.dtsi                  |  1 -
>>>>>>   arch/arm/boot/dts/dm814x.dtsi                  |  1 -
>>>>>>   arch/arm/boot/dts/dra7.dtsi                    |  1 -
>>>>>>   drivers/net/ethernet/ti/cpsw.c                 | 13 +++----------
>>>>>>   drivers/net/ethernet/ti/cpsw.h                 |  1 -
>>>>>>   drivers/net/ethernet/ti/davinci_cpdma.c        |  6 ++++++
>>>>>>   drivers/net/ethernet/ti/davinci_cpdma.h        |  1 +
>>>>>>   9 files changed, 10 insertions(+), 16 deletions(-)
>>>>>>
>>>>>
>>>>
>>>
>>
>

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