linux-kernel - Re: [PATCH v3] serial: mxs-auart: keep the AUART unit in reset state when not in use

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Date:	Thu, 16 Jul 2015 17:36:17 +0200
From:	Stefan Wahren <stefan.wahren@...e.com>
To:	Juergen Borleis <jbe@...gutronix.de>, linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-serial@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, kernel@...gutronix.de
Subject: Re: [PATCH v3] serial: mxs-auart: keep the AUART unit in reset state
 when not in use

> Whenever the UART device driver gets closed from userland, the driver
> disables the UART unit and then stops its clock 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() and re-enabling of the unit's clock.
>
> In this case we end up receiving an additional bogus character
> immediately.
>
> The solution in this change is to move the AUART unit into its reset
> state on close() and only release it from its reset state on the next
> open().
>
> Signed-off-by: Juergen Borleis <jbe@...gutronix.de>
> ---

Acked-by: Stefan Wahren <stefan.wahren@...e.com>

Thanks
Stefan
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