From mboxrd@z Thu Jan  1 00:00:00 1970
From: viresh.kumar@st.com (Viresh KUMAR)
Date: Wed, 26 May 2010 13:57:05 +0530
Subject: Should we pass amba device peripheral id with device structure
	or not?
In-Reply-To: <20100524213237.GH21117@n2100.arm.linux.org.uk>
References: <4BECF57A.4050802@st.com>
	<20100521193802.GG11042@n2100.arm.linux.org.uk>
	<4BFA02DC.6090906@st.com>
	<20100524213237.GH21117@n2100.arm.linux.org.uk>
Message-ID: <4BFCDB59.5060304@st.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On 5/25/2010 3:02 AM, Russell King - ARM Linux wrote:
> On Mon, May 24, 2010 at 10:08:52AM +0530, Viresh KUMAR wrote:
>> On 5/22/2010 1:08 AM, Russell King - ARM Linux wrote:
>>> On Fri, May 14, 2010 at 12:32:18PM +0530, Viresh KUMAR wrote:
>>>> amba_device_register function reads and updates peripheral id from
>>>> hardware registers, whenever we register any amba device. If clock
>>>> to device is disabled, then amba_device_register will not be able
>>>> to read and update this value.
>>>
>>> This is a potential problem - if the drivers are already initialized
>>> in the kernel, then the drivers will try to initialize as soon as
>>> amba_device_register() is called.  If the registers aren't accessible
>>> at amba_device_register() time, the driver initialization could fail.
>>>
>>> I think it's better to understand what's going on here before making
>>> suggestions.
>>>
>>> The clks in the primecell drivers are for the external side clocks
>>> only; these drivers all make the assumption that the AMBA bus clock
>>> is always enabled.  Does your SoC turn the AMBA bus clock to peripherals
>>> on and off?
>>
>> There is only one bit per peripheral to enable/disable clock.
>> So with clocks disabled, we get 0x00000000 on read from device registers.
> 
> So... that must mean your hardware gates both the peripheral clock and
> the per-primecell bus clock together.  Let's hope that the bus clock
> control takes notice of any in-progress bus transaction...
> 
> However, I'm still concerned - the driver's use of clk_enable/clk_disable
> is based on the assumption that these calls do not affect the bus clock -
> we expect to be able to write to registers before the first clk_enable()
> call.
> 
> And as I've said (and you cut off of the quote) if we have SoCs where the
> bus clock is controllable, we need amba/bus.c to deal with that situation.
> 
> Okay, I'll look at addressing that _after_ this merge window has closed.
> 

OK.