netdev - Re: [PATCH v2] sh_eth: use RNC mode for R8A7790/R87791

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Message-ID: <538CFA08.3010302@cogentembedded.com>
Date:	Tue, 03 Jun 2014 02:26:16 +0400
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	David Miller <davem@...emloft.net>
CC:	ben.dooks@...ethink.co.uk, linux-kernel@...ethink.co.uk,
	netdev@...r.kernel.org, nobuhiro.iwamatsu.yj@...esas.com,
	magnus.damn@...nsource.se, horms@...ge.net.au,
	yoshihiro.shimoda.uh@...esas.com, cm-hiep@...so.co.jp,
	linux-sh@...r.kernel.org
Subject: Re: [PATCH v2] sh_eth: use RNC mode for R8A7790/R87791

Hello.

On 06/03/2014 01:17 AM, Sergei Shtylyov wrote:

>>>>>      Looks like the early SH2/3 SoCs didn't implement the whole register.
>>>>> Despite that, sh_eth_dev_init() always writes to this register... :-/
>>>>> So far, the RMCR.RNC bit was mostly set for the Gigabit-capable
>>>>> controllers, however that rule wasn't strictly followed. Well, this
>>>>> driver is still a mess, and it's hard to deal with it without the
>>>>> necessary documentation.

>>>> Why don't we therefore:

>>>> 1) Skip the register write if the per-chip value is zero.

>>>     I rather thought about not writing when the register is not
>>>     implemented.
>>> I'll probably look into this when I have time.

>>>> 2) Add the RNC bit to all of the gigabit capable controllers.

>>>     I probably misspoke -- all the Gigabit controllers already have it
>>>     set, it's just that some 100 MBbps ones have it set, but most don't.

>> So these chips that do not implement the register, they only process
>> one RX descriptor at a time until the interrupt handler re-enables
>> DMA receive?

>     I just don't know. Looks like the driver is broken on SH2/3 even more than
> I thought: it always reads the EDRRR register in sh_eth_rx() trying to
> understand if the reception has been stopped but that register doesn't seem to
> exist on SH2/3. Moreover, sh_eth_interrupt() reads EESR in order to determine
> the interrupt status but that register doesn't seem to exist on SH2/3 either!

    OK, I've chased down the commit that broke SH2/3 support 3+ years ago; 
here it is:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4a55530f38e4eeee3afb06093e81309138fe8360

    All the registers I've mentioned did exist on SH2/3, they just got missed 
in the mapping arrays.

WBR, Sergei

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