linux-kernel - Re: [PATCH v4 2/2] serial: mxs-auart: keep the AUART unit in reset state when not in use

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Date:	Fri, 07 Aug 2015 11:30:27 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Juergen Borleis <jbe@...gutronix.de>, linux-kernel@...r.kernel.org
CC:	Stefan Wahren <stefan.wahren@...e.com>,
	linux-serial@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	kernel@...gutronix.de, Jiri Slaby <jslaby@...e.cz>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 2/2] serial: mxs-auart: keep the AUART unit in reset
 state when not in use

On 08/07/2015 06:47 AM, Juergen Borleis wrote:
> Whenever the UART device driver gets closed from userland, the driver
> disables the UART unit and then stops the clocks to save power.
> 
> The bit which disabled the UART unit is described as:
> 
>  "UART Enable. If this bit is set to 1, the UART is enabled. Data
>   transmission and reception occurs for the UART signals. When the
>   UART is disabled in the middle of transmission or reception, it
>   completes the current character before stopping."
> 
> The important part is the "it completes the current character". Whenever
> a reception is ongoing when the UART gets disabled (including the clock
> off) the statemachine freezes and "remembers" this state on the next
> open() when re-enabling the unit's clock.
> 
> In this case we end up receiving an additional bogus character
> immediately.
> 
> The solution in this change is to switch the AUART unit into its reset
> state on close() and only release it from its reset state on the next
> open().
> 
> Note: when the unit is also used as system console it is always 'in use',
> so we cannot reset it on close().

The code is still a bit ackward but looks functional, so

Reviewed-by: Peter Hurley <peter@...leysoftware.com>


PS - This mxs driver has the same problem as the i.MX driver.
     The irq request and hardware init should be done in startup().

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