Re: [PATCH v2 0/2] net: ethernet: ti: cpsw: delete rx_descs property
From: Ivan Khoronzhuk
Date: Tue Jun 14 2016 - 08:26:54 EST
On 13.06.16 11:22, 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.
Yes.
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.
This patch-set doesn't forbid it. It solves only masked issue.
In case if "DDR", there is question, can happen that not every version
does support it (https://patchwork.kernel.org/patch/7360621/)
But, anyway, it should be done with separate series.
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.
Not sure about DT, but I agree, there should be separate parameter like
pool size.
So based on no_bd_ram DT entry, the driver can decide whether it can use
internal BD-ram or DDR to initialize the cpdma driver.
Regards
Mugunthan V N
--
Regards,
Ivan Khoronzhuk