RE: [PATCH] net: use hardware buffer pool to allocate skb

["Jiafei.Pan@freescale.com" <Jiafei.Pan@freescale.com>]

> -----Original Message-----
> From: Alexander Duyck [mailto:alexander.h.duyck@redhat.com]
> Sent: Thursday, October 16, 2014 11:28 PM
> To: Pan Jiafei-B37022; Eric Dumazet
> Cc: David Miller; jkosina@suse.cz; netdev@vger.kernel.org; Li Yang-Leo-R58472;
> linux-doc@vger.kernel.org
> Subject: Re: [PATCH] net: use hardware buffer pool to allocate skb
> 
> 
> On 10/15/2014 10:15 PM, Jiafei.Pan@freescale.com wrote:
> >> -----Original Message-----
> >> From: Eric Dumazet [mailto:eric.dumazet@gmail.com]
> >> Sent: Thursday, October 16, 2014 12:15 PM
> >> To: Pan Jiafei-B37022
> >> Cc: David Miller; jkosina@suse.cz; netdev@vger.kernel.org; Li Yang-Leo-R58472;
> >> linux-doc@vger.kernel.org
> >> Subject: Re: [PATCH] net: use hardware buffer pool to allocate skb
> >>
> >> On Thu, 2014-10-16 at 02:17 +0000, Jiafei.Pan@freescale.com wrote:
> >>
> >>> Thanks for your comments and suggestion. In my case, I want to build skb
> >>> from hardware block specified memory, I only can see two ways, one is
> modified
> >>> net card driver replace common skb allocation function with my specially
> >>> functions, another way is to hack common skb allocation function in which
> >>> redirect to my specially functions. My patch is just for the second way.
> >>> Except these two ways, would you please give me some advice for some other
> >>> ways for my case? Thanks
> >> I suggest you read drivers/net/ethernet numerous examples.
> >>
> >> No need to change anything  in net/* or include/*, really.
> >>
> >> For a start, look at drivers/net/ethernet/intel/igb/igb_main.c
> >>
> >> Mentioning 'hack' in your mails simply should hint you are doing
> >> something very wrong.
> >>
> >> What makes you think your hardware is so special ?
> >>
> > In fact, I am developing a bridge driver, it can bridge between any other the
> > third party net card and my own net card. My target is to let any other the
> > third party net card can directly use my own net card specified buffer, then
> > there will be no memory copy in the whole bridge process.
> > By the way, I don’t see any similar between igb_main.c and my case. And also
> > My bridge also can’t implemented with "skb frag" in order to aim at zero
> memory
> > copy.
> 
> I think the part you are not getting is that is how buffers are
> essentially handled now.  

[Pan Jiafei] Hi, Alex, thanks for your comments. I don’t confirm that
you have catch my concerns. For example, I want to add igb net card 
into my bridge, and want to igb net driver allocate skb by using
my specified memory address, but I don’t want to modify igb net driver
directly, how to do this in my bridge drivers?

Thanks,
Jiafei.

So for example in the case if igb the only
> part we have copied out is usually the header, or the entire frame in
> the case of small packets.  This has to happen in order to allow for
> changes to the header for routing and such.  Beyond that the frags that
> are passed are the buffers that igb is still holding onto.  So
> effectively what the other device transmits in a bridging/routing
> scenario is my own net card specified buffer plus the copied/modified
> header.
> 
> For a brief period igb used build_skb but that isn't valid on most
> systems as memory mapped for a device can be overwritten if the page is
> unmapped resulting in any changes to the header for routing/bridging
> purposes being invalidated.  Thus we cannot use the buffers for both the
> skb->data header which may be changed and Rx DMA simultaneously.
> 
> Thanks,
> 
> Alex