linux-kernel - Re: [PATCH 3/8] dmaengine: ste_dma40: Actually write the runtime configuration to registers

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Message-ID: <CAH+eYFB2+oMPXsnzQVVrEK8CjKGVYFa_n4Wxr=CpdtJwFfOGYw@mail.gmail.com>
Date:	Mon, 15 Apr 2013 13:36:00 +0200
From:	Rabin Vincent <rabin@....in>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	arnd@...db.de, linus.walleij@...ricsson.com
Subject: Re: [PATCH 3/8] dmaengine: ste_dma40: Actually write the runtime
 configuration to registers

2013/4/15 Lee Jones <lee.jones@...aro.org>:
> On Fri, 12 Apr 2013, Rabin Vincent wrote:
>> 2013/4/9 Lee Jones <lee.jones@...aro.org>:
>> > Someone has spent a fair amount of effort writing a runtime configuration
>> > changing algorithm for DMA clients. However, the config appears to never
>> > actually make it to hardware. In order for the configuration to take hold
>> > we need to issue a d40_config_write(), as this is the routine which writes
>> > it into the hardware's registers.
>>
>> No, it's not.  This function is only for initial configuration which
>> should only be written when the channel is allocated.  In fact, by
>> calling it here in runtime_config, you are introducing a serious bug:
>> other logical channels on the same physical channel will stop because of
>> the SSLNK/SDLNK of the physical channel being zeroed.
>>
>> The runtime config already makes it the hardware in the existing code,
>> via d40_*_cfg().
>
> Sorry Rabin, but the only place I can see the config being written is
> in d40_config_write().
>
> Can you paste the line of code in d40_*_cfg() which actually writes
> the config to hardware please? I don't see it.

It's not that simple.  There are some pointers passed to d40_*_cfg() and
that function writes the configuration to the variables those pointers
point to (d40c->log_def.lcsp1, d40c->src_def_cfg, etc.).  Please read
the code to see how those variables end up being used later when the
LLIs are prepared for the HW.
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