Subject: Re: [PATCH (net.git) 2/4] stmmac: fix and better tune the default buffer sizes

From: Giuseppe CAVALLARO <[email-masked]>
Date: Thu, 27 Feb 2014 14:54:17 +0100

Hi David

On 2/27/2014 2:31 PM, David Laight wrote:
> From: Giuseppe CAVALLARO
> ...
>>> Also (provided the hardware supports it) the rx buffers (are these
>>> the ones being sized?) need to be aligned on a 4n+2 boundary in
>>> order to avoid a realignment copy later on.
>>
>> This is true and indeed I had added the STMMAC_ALIGN to align all.
>> In the past to get the right alignment for SH4.
>>
>>> So I'm not sure that some of these sizes are right and/or optimal.
>>
>> What do you suggest?
>>
>> Maybe, I can use a default for sure < 4KiB suitable to be used for VLAN
>> frames (it will be aligned later).
>
> Dunno... It rather depends on what the length is actually used for!
> What you don’t want to be doing is adding 2 (for the 4n+2) and then
> mallocing a 4096+2 byte buffer somewhere.

thanks for your support.

What do you mean for 4n+2?

we reserve 2 more when allocate the skb.
I had seen that, When I used 2KiB as default this pushes us to allocate
from the next larger slab on old SH4 platforms and sometime forcing me
to increment the min_free_kbytes also when we work with the std
Ethernet MTU :-(.

So for sure 4KiB is really big and not good especially for embedded
system where this driver lives.

>
> If the hardware does receive desegmentation, then you need to handle
> the 64k+ receives somewhere.
> If it doesn't then it doesn't matter if the hardware rx buffer size is
> slightly too large (eg for VLAN or encapsulation full sized frames in PPoE).
> 1536 bytes for the memory buffer avoids cache line sharing (read to
> offset 2).

IIUC, so what you finally suggest is to use a default value w/o Koption
and 1536bytes is suitable for vlan etc. This is ok and can be managed
w/o breaking the compatibility with old mac where the rx hw buffer
are limited in size and where jumbo is not supported.

> The last ethernet driver I wrote from scratch (maybe 20 years ago) set
> the rx-ring to point to an array of 512 byte buffers (last was shorter
> to avoid an extra page) and did an aligned copy into the message buffer.
> Only frames that crossed the ring end needed two copies.
> ISTR making the copy be cache line aligned so that a special cache line
> copy function could be used (I don't know if it ever was).
> For that system the cost of the aligned data copies was less that the
> complexity and cost of setting up the iommu.

ok

peppe
> 	David