linux-kernel - Re: [PATCH] spi: tegra: add spi driver for SLINK controller

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Message-ID: <508ED121.1050702@wwwdotorg.org>
Date:	Mon, 29 Oct 2012 12:55:29 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Laxman Dewangan <ldewangan@...dia.com>
CC:	"broonie@...nsource.wolfsonmicro.com" 
	<broonie@...nsource.wolfsonmicro.com>,
	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"spi-devel-general@...ts.sourceforge.net" 
	<spi-devel-general@...ts.sourceforge.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] spi: tegra: add spi driver for SLINK controller

On 10/29/2012 10:18 AM, Laxman Dewangan wrote:
> On Monday 29 October 2012 08:47 PM, Stephen Warren wrote:
>> On 10/26/2012 12:49 PM, Laxman Dewangan wrote:
>>>>
>>>> Why not just always set SLINK_FIFO_ERROR; does it have to be set in the
>>>> write only if the status was previously asserted? If that is true, how
>>>> do you avoid a race condition where the bit gets set in SLINK_STATUS
>>>> after you read it but before you write to clear it?
>>> Status gets updated together. There is no steps of updating status.
>> Sorry, I don't understand this answer.
> 
> The status should be updated once by HW and so there is no race condition.
> HW behavior is that if the tx or Rx error occurs, it updates the status,
> generates interrupt and still continue transfer and later on, it
> generates the ready.
> In first isr, we read status, error status found and so in isr thread,
> we reset controller to stop the transfer.
> 
> So in good state, only ready bit will be set and hence writing 1 to
> clear it.
> In error state, clearing error first in ISR and in isr thread resetting
> the controller to stop the controller engine.

OK, I see why there's no race. It still seems simply to me if
tegra_slink_clear_status() just always writes all the status bits, but I
suppose it isn't a correctness issue.

>>> Is there a way to support the reset of controller. We will need this
>>> functionality.
>>
>> Why do we need to reset the controller at all; can't we simply program
>> all the (few) configuration registers? Are there HW bugs that hang the
>> controller and require a reset or something?
> 
> HW generates error,  then interrupt and still continue transfer and
> later point of time it generates the transfer done.
> We want to stop the transfer once error get detected. For this we need
> to reset controller.
> I did disabling rx and tx but still controller shows as busy.

Oh dear. Well, I guess it'll have to be OK then; we'll just have to find
a way of decoupling this API from the mach-tegra directory later:-(
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