From: andre.przywara@arm•com (Andre Przywara)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH 10/10] drivers: PL011: add support for the ARM SBSA generic UART
Date: Wed, 04 Mar 2015 17:47:40 +0000 [thread overview]
Message-ID: <54F7453C.5060407@arm.com> (raw)
In-Reply-To: <20150217161613.GC3783@e103592.cambridge.arm.com>
Philip,
sorry for the late reply, that was stuck in my Drafts folder :-(
On 02/17/2015 04:16 PM, Dave P Martin wrote:
> On Tue, Feb 17, 2015 at 10:55:35AM -0500, Philip Elcan wrote:
>> On 01/16/2015 12:23 PM, Andre Przywara wrote:
>>> The ARM Server Base System Architecture[1] document describes a
>>> generic UART which is a subset of the PL011 UART.
>>> It lacks DMA support, baud rate control and modem status line
>>> control, among other things.
>>> The idea is to move the UART initialization and setup into the
>>> firmware (which does this job today already) and let the kernel just
>>> use the UART for sending and receiving characters.
>>> We use the recent refactoring the build a new struct uart_ops
>>> variable which points to some new functions avoiding access to the
>>> missing registers. We reuse as much existing PL011 code as possible.
>>>
>>> In contrast to the PL011 the SBSA UART does not define any AMBA or
>>> PrimeCell relations, so we go a pretty generic probe function
>>> which only uses platform device functions.
>>> A DT binding is provided, but other systems can easily attach to it,
>>> too (hint, hint!).
>>>
>>> Signed-off-by: Andre Przywara <andre.przywara@arm•com>
>>> ---
>>
>> <snip>
>>
>> Andre,
>>
>> I'm a little late to address this patchset, but the SBSA defines all
>> the Generic UART registers 32-bit wide. However, the amba-pl011 driver
>> uses 16-bit accessors. How will you be handling that? Can the ARM PL011
>> hardware handle 32-bit access?
>
> Interesting question. The PL011 TRM [1] specifies only a 16-bit-wide
> APB bus interface, but does not say that 32-bit accesses won't work.
>
> I suspect that 32-bit accesses will work on all or most PL011s -- if
> that looks too risky or we can't find enough test platforms to be sure
> of this, then we could maybe abstract the register access size as a
> quirk.
>
> Andre may already have an answer on this.
I am not sure if the 32-bit register _width_ mentioned in the spec
really mandates 32-bit accesses, also the width may be just a spec bug.
Since the SBSA states that an ARM PL011r1p5 is a valid SBSA-UART
implementation, I wonder how this goes together. Also the highest
non-reserved bit in the registers is bit 15 in PL011 and bit 11 in the
SBSA subset.
I fear the actual bus connection is an implementation detail. Given the
fact that all existing PL011 hardware so far works with the 16bit
accesses, I don't dare to change this.
So I'd suggest to keep it as readw/writew for now and the revisit this
topic if some SBSA UART users complain. That makes it easier to justify
the rather invasive change to all MMIO accessors (which I already tried
on one machine for now without problems, btw).
Cheers,
Andre.
next prev parent reply other threads:[~2015-03-04 17:47 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-16 17:22 [PATCH 00/10] drivers: PL011: add ARM SBSA Generic UART support Andre Przywara
2015-01-16 17:22 ` [PATCH 01/10] drivers: PL011: avoid potential unregister_driver call Andre Przywara
2015-01-16 17:22 ` [PATCH 02/10] drivers: PL011: refactor pl011_startup() Andre Przywara
2015-01-16 17:22 ` [PATCH 03/10] drivers: PL011: refactor pl011_shutdown() Andre Przywara
2015-01-16 17:23 ` [PATCH 04/10] drivers: PL011: refactor pl011_set_termios() Andre Przywara
2015-01-16 17:23 ` [PATCH 05/10] drivers: PL011: refactor pl011_probe() Andre Przywara
2015-01-16 17:23 ` [PATCH 06/10] drivers: PL011: replace UART_MIS reading with _RIS & _IMSC Andre Przywara
2015-01-16 17:23 ` [PATCH 07/10] drivers: PL011: move cts_event workaround into separate function Andre Przywara
2015-01-16 17:23 ` [PATCH 08/10] drivers: PL011: allow avoiding UART enabling/disabling Andre Przywara
2015-01-16 17:23 ` [PATCH 09/10] drivers: PL011: allow to supply fixed option string Andre Przywara
2015-01-16 17:23 ` [PATCH 10/10] drivers: PL011: add support for the ARM SBSA generic UART Andre Przywara
2015-01-16 17:34 ` Mark Rutland
2015-01-16 18:07 ` Andre Przywara
2015-01-16 18:12 ` Mark Rutland
2015-01-16 18:33 ` Andre Przywara
2015-01-16 18:37 ` Mark Rutland
2015-01-19 13:31 ` Arnd Bergmann
2015-01-19 13:44 ` Andre Przywara
2015-01-19 13:56 ` Arnd Bergmann
2015-02-17 15:55 ` Philip Elcan
2015-02-17 16:16 ` Dave Martin
2015-03-04 17:47 ` Andre Przywara [this message]
2015-03-05 11:15 ` Dave Martin
2015-01-16 17:31 ` [PATCH 00/10] drivers: PL011: add ARM SBSA Generic UART support Arnd Bergmann
2015-01-16 17:53 ` Andre Przywara
2015-01-20 13:08 ` Graeme Gregory
2015-01-20 13:55 ` Andre Przywara
2015-01-20 14:26 ` Graeme Gregory
2015-01-20 14:33 ` Andre Przywara
2015-01-20 14:52 ` Graeme Gregory
2015-01-21 9:26 ` Graeme Gregory
2015-01-20 14:32 ` Dave P Martin
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